To the Fun Science Gallery Contents


1 - Small Freshwater Organisms

Giorgio Carboni, December 2006
Translation edited by
Sarah Pogue, December 2009



    Blue-green algae
    Green algae
    Where to collect Protists
    Raising Protists
    Raising haematococcus
    How to observe Protists
Small multicellular organisms
    Sponges, jellyfishes, hydra
    Flatworms and annelida
    Water snails
    Aquatic insects
    Beneath the river pebbles
    Water spiders
    Eggs and larvae of amphibians

Figure 1 - Cysts of Haematococcus
(Haematococcus pluvialis). Dark Field.



This article is the first in a series devoted to observations using a microscope and deals with the microscopic forms of life that inhabit mainly freshwater ecosystems. The aquatic environments host a rich variety of organisms and are a wonder to behold through a microscope. Before beginning your observations, you should read the chapter on systematics and the classification of living beings in a biology textbook. Keep near at hand a list of the different categories to better understand where the different organisms you will meet are placed. As there is no agreement on the classification of the Protists, if you read different books draw a diagram to show how each text classifies these organisms. For the sake of convenience, I remind you of the principal categories of living beings: Kingdom, Division or Phylum, Class, Order, Family, Genus, Species. The category of Division is used for plants only, while at the same level Phylum is used for animals.

In the following article you can watch films of Protists and other pond organisms:


In aquatic environments such as ponds, lakes, rivers and oceans innumerable living beings have their home. In this article, we will consider mainly the freshwater environment, and in particular stagnant waters, which are among the richest environments in terms of life forms. By taking small samples of water, it is possible to observe microscopic organisms which consist of only one cell and also miniscule or even microscopic multicellular organisms.

To observe Protozoa and unicellular Algae with a microscope is similar to undertaking a journey to another planet. Their form, their behaviour and their habits are so strange that you will be shocked. For instance, you would expect that the microscopic Algae, distant ancestors of plants, would live fixed to the bottom as if they had roots. On the contrary, you can see many of them swimming rapidly. In fact, many of them are equipped with one or two or even more flagella. Moreover, it is useless for you to search for those flagella behind their body as if they were the propeller of a motorboat, because in almost all cases the Algae have their flagella in front.

In order to start your explorations look for some stagnant water. For this purpose ponds, basins, ditches, puddles, saucers, fountains, water containers for plants, drums for rain water and aquaria work well. If the water is green in colour or if it has heaps of disgusting looking greenish stuff in it, this means it is very rich in life. While ordinary people are horrified by this "disgusting" water, a microscopist, who is not just an ordinary person, is instead irresistibly attracted to it. In fact, the observation of the microscopic inhabitants of these ecosystems is so fascinating you could continue it every days for hours.

BACTERIA (Kingdom)

The smallest organisms it is possible to observe with an optical microscope are the bacteria (Figures 2 and 3). You can find them in any sample of water, but they are present in larger quantities in waters rich in decomposing organic substances. Bacteria are formed of only one cell and this is why they are called unicellular. Bacteria have a simple cellular organization: their genetic material is dispersed in the cytoplasm and for this reason they are named prokaryota (proto nucleus). Bacteria are nearly transparent and they are best seen with special illumination systems such as phase contrast and differential interference contrast (DIC). The Blue-Green Algae or Cyanophyceae are counted amongst the Bacteria. They were one of the first organisms to appear on Earth and they are able to produce their own food by means of photosynthesis. In Figure 2, you can see some bacteria (small and clear) and a blue-green alga (large and with a segmented appearance). To best distinguish the bacteria, you can watch the film indicated in [1004].


Figure 2 - Bacteria and blue-green alga.
(Phase contrast) L = 0.084 mm.

Figure 3 - Blue-green alga (in center) and
diatom (above). (Phase contrast) L = 0.21 mm.

Figure 4 - Nostoc (blue-green alga) coloured
with haematoxylin. L = 0.21 mm.


Blue-Green Algae (division: Cyanophyta)
Most likely, the first organisms to appear on Earth and which were able to produce their own food by means of photosynthesis were the Blue-Green Algae. They are also known as Cyanophyceae because of their blue-green colour. Due to their cellular organisation, these organisms are halfway between Bacteria and Protists and they are called eubacteria. These algae are often gathered into little groups of 2-4 cells or even of many cells immersed in a gelatinous substance. At other times, they are arranged into necklaces of cells which move slowly and often in an oscillating manner. You can easily find them mixed with filamentous algae like the spirogyra. These and other algae are formed of many individuals, which nevertheless are not differentiated. For this reason, these organisms are not considered to be multicellular, but only colonial. The organisms we will meet later are all eukaryotic.

PROTISTA (Kingdom)

As with bacteria, Protists are also formed of only one cell, but unlike the bacteria, Protists have a much more complex cellular organisation and are much greater in size. In particular, while the genetic material of bacteria is dispersed in the cytoplasm, that of Protists is gathered inside a membrane to form a nucleus. For this reason, Protists are called eukaryotes (true nucleus). Plants, mushrooms and animals came from Protists as a result of a long evolutionary process and they are also made up of eukaryotic cells. Thus, Protists remind us of primitive life forms, even if they actually continued to evolve and attained astounding levels of complexity indeed as unicellular organisms.

It is possible to divide Protists into unicellular Algae and Protozoa. Protozoa feed on organic detritus and on other organisms like bacteria and other Protists and so they are called "heterotrophs". Unicellular Algae, instead, produce their own food through photosynthesis and they are called "autotrophs". As Protozoa can have features similar to those of algae and vice versa, scientists call these living beings: "Protists". Hence, do not consider Protozoa as "proto animals" and the unicellular algae as "proto plants", because each species often shares animal and vegetable characteristics. To have some further information on Protists and to see some films of these organisms, visit the Protist Park [1003].


Figure 5 - Euglena sp. ( 400 X ca).

Figure 6 - Phacus sp. (400 X ca).

Figure 7 - Anisonema sp.
(Phase contrast. 400 X ca).


Euglenids (division: Euglenophyta)
Usually, Euglenids are photosynthetic organisms which move by means of flagella. As with other Protists, these organisms seem to do all they can to surprise us. Anisonema (Figure 7) is an alga lacking in chloroplasts which feeds on organic detritus. It has two flagella, the shorter of which is very mobile and directed forwards. The other flagellum passes under the organism which drags it along as though it were an inert tail. The Peranema has also lost its chloroplasts and is equipped with only one long flagellum that is oriented forwards. As the organism moves, its flagellum vibrates as if it were exploring the environment. Euglena (Figure 5) is widespread, it has a large number of chloroplasts and an orange coloured stigma that is sensitive to light and helps the Protist to locate more illuminated places. The body of Euglena has some helicoidal striping and is very mobile. Astasia klebsi is similar to euglena, but lacks chloroplasts and also feeds on other Protozoa. It seems to amuse itself in a very particular way by squeezing itself like a sole in order to pass under the filaments of spirogyra. It has a very plastic body and when it has succeeded in squeezing itself to pass through a narrow opening, it likes very much to stretch with a series of contractions.


Figure 8 - Haematococcus pluvialis. (Phase contrast).

Figure 9 - Pediastrum sp.

Figure 10 - Scenedesmus obliquus.
(Phase contrast).

Green Algae (division: Chlorophyta)
Due to the chemical composition of the pigments of the chloroplasts and starches and for other similarities, Green Algae, and in particular the Chaetosporales, are considered the progenitors of the superior plants. Green Algae include thousands of unicellular species. Among the more numerous and beautiful orders is that of the Coccoid Algae (Chlorococcales). Some of them form colonies made up of many individuals, for example the Pediastrum, which has a pretty starlike form and Scenedesmus, whose members arrange themselves side by side in little groups. Other green algae swim freely by means of flagella. This is the case of the Haematococcus pluvialis, a graceful spherical alga with a large yellow-green, pear-shaped chloroplast in its centre (Figure 8). From the tip of this chloroplast two flagella protrude which, when beaten, cause the cell to vibrate in a peculiar manner. When these Protists encounter difficulties, they fix themselves to the bottom, their chloroplast enriches itself with carotenoids and they become deep red in colour. The chloroplast grows in size and often it almost completely fills the cell. In his garden, the author has a white plastic drum to collect rain water which often stains spontaneously with a red colour because of a layer of Haematococcus. It is possible to breed this pretty alga as described further on.

The order of Volvocal Algae (Volvocales) comprises striking colonial forms such as the Gonium, the Pandorina, the Eudorina and the magnificent Volvox, a spherical colony made up of thousands of flagellate cells.

Once more amongst the Green Algae, we have the class of the Conjugate (Coniugatophyceae), where we meet the order of the Desmids (Desmidiales), enchanting organisms to be seen with the microscope because of their symmetrical appearance. As you can see in the Cosmarium (Figure 11), their cell is nearly separated by a median narrowing into two identical semi-cells. The Closterium looks like a crescent moon (Figure 12) and moves slowly, by raising the back and then the tips.


Figure 11 - Cosmarium sp.

Figure 12 - Closterium.

Figure 13 - Spirogyrae in conjugation.


With the order of the Zygnemales, the Conjugates again have some surprises reserved. Spirogyra belongs to this group (Figure 13), a filamentous alga which often invades stagnant waters and whose heaps look extremely disgusting, especially if they contain many bubbles and form a foam. Frogs are very fond of croaking on that greenish bed. However, if you observe the Spirogyrae with the microscope you will undoubtedly be enchanted. In fact, these long and filamentous algae look like transparent reeds, each segment of which is a cell. As they are transparent, inside each cell you can see the chloroplasts arranged in elegant helixes that, depending on the case, can have one or more beginnings. On these algae live many Protists, some of which slide on the surface and others which are attached. A forest of spirogyra hides a variety of microscopic forms of life.


Figure 14 - Diatoms (30 forms Kemp' slide).
Field = 1 mm.

Figure 15 - Cymbella inaequalis. L = 103 μ.
(by Alessandro Bertoglio [1006], [1007]).

Figure 16 - Triceratium nobile. L = 83 μ.
(by Alessandro Bertoglio [1006], [1007]).


Diatoms (class: Bacillariophyceae)
Diatoms are algae too, but unlike the Astasia which has a very changeable body, diatoms have a fixed form (Figures 14, 15 and 16). In fact, they are contained in a case made up of a microscopic silica box closed by a slightly larger lid. They move forward by sliding along in a straight line, then they stop, shake slightly and restart in the opposite direction without any rotation, as though they have gone into reverse. But, if they are closed in a case how do they manage to move? The box in which they live (frustule) has a lot of little holes and on the bottom it has a long fissure called a raphe. Through this fissure some protoplasm comes out which is then reabsorbed by the other part, so this delicate creature glides along by means of a tiny track as if it were a tank. The little pores, grooves and reliefs arranged in a regular pattern form pretty grids on the frustules of diatoms. These algae are so beautiful that many microscopists deal only with them. To obtain diatoms, look for them on the bottom of a puddle with a pipette, being careful not to take up mud aswell. On the bottom of an aquarium or of a glass jar in which you sustain a culture of Protists, it is possible to pick up diatoms without also collecting mud and sand.


Figure 17 - Amoeba. (Phase contrast).

Figure 18 - Heliozoa. Diameter = 20 µ.

Figure 19 - Ciliates during division.
(From: "Amici del microscopio" [1008])


Amoeba (class: Rhizopoda)
The principal orders of this class are: Amoebida (amoeba) and Testacea (thecamoeba). Whilst Diatoms have a stiff form, amoeba have a very mobile form, due to a very thin membrane which allows them to change shape continuously. However, when you encounter an amoeba, you will without doubt recognise it immediately. Amoebae (Figure 17) move slowly while expanding a pseudopodia first in one direction and then in another. Everything in its body rolls. It is as though you were lying down and in order to advance you rolled the heart, liver, lungs and all the other organs over each other inside a transparent and shapeless body. A flux of organelles head in the direction in which you see a pseudopodium form, while at the same time, another pseudopodium directed in a different direction can develop. After a period of uncertainty, one flux slows down and the other prevails. Nobody knows where the amoeba has its nose, but it is able to perceive the presence of an attractive microorganism which it tries to phagocytize by surrounding it with its pseudopodia. There are many species of amoeba, some of which give out only one pseudopodium at a time, while others emit many of them even upwards and with a pointed form. Thecamoeba are amoeba that build a shell in which to live. An opening allows them to jut out of the shell and to advance. Amoeba also live in the bottom of the water body, on decomposing leaves, on spirogyra, etc.

Heliozoa (class: Actinopoda)
Heliozoa belong to the class of Actinopoda and to the order Heliozoa (Figure 18). These are small organisms from whose bodies a lot of axopods radiate, thin and poisonous rays they use to capture and phagocytize microorganisms. The shape of these protists resembles the Sun and gives them their name.

Ciliates (class: Ciliata)
Ciliates form a community which is very species-rich. Their main characteristic is a body covered with many cilia (Figure 19). These cilia are often fused into cirri or grouped into long membranelles. Usually, cilia are moved in a coordinated way with a wave-like motion and are used to swim. Some ciliates also use the cirri one at the time for "walking" on solid surfaces. The membranelles look like mobile fences and are usually used to draw in food particles. The structure of these cilia, cirri and membranelles is described in many biology textbooks such as the [106]. Whereas the cilia move due to the longitudinal sliding of microtubules over one another, the flagella of many species of bacteria are stiff and they rotate as a result of an electric biological engine located in the point of insertion of the flagellum into the membrane. These bacteria are able to regulate the rotating speed of the flagellum. Ciliates feed on bacteria, on other Protists and on organic detritus. They are divided into three orders: Holotricha, whose cilia are uniformly distributed on the body (paramecium, euplotes, coleps, etc.); Peritricha, which have a wide cytostomatic opening (mouth) surrounded by a membranelle (vorticella, stentor, etc.); and Spirotricha, which have a membranelle along a narrow cytostomatic opening.

The best known ciliate is the paramecium (Figure 20), whose form resembles that of a slipper, but unlike the slipper paramecium is always in motion, occupied in an incessant search for food. It is fascinating to see an euplotes that moves around a spirogyra looking for bacteria and organic detritus, particularly if you notice that is able to swim using the hundreds of cirri with which it is equipped and also to walk by moving its ventral cirri one at a time. Coleps is a ciliate with a typical barrel form that swims quickly although it sometimes lingers around food.


Figure 20 - Paramecium.

Figure 21 - Vorticella. (Phase contrast).

Figure 22 - Stentor.

What is there to say then about vorticellae (Figure 21), a bell-shaped protist equipped with a row of oscillating hairs around its oral opening that create a vortex in the water to draw in food particles. These microorganisms remain attached to the bottom by means of a retractile stalk. If it is disturbed, vorticella suddenly contracts itself into a sphere and its stalk contracts by taking the shape of a spring. After a while, vorticella stretches out its stalk, the bell opens up again and the hairs restart. During your observations, you may also come across green vorticellae with symbiotic Algae inside their bodies. You can also see colonies of vorticella which are particularly striking to observe. Every so often, vorticellae leave the substratum and swim freely. The famished and trumpet-shaped Stentor has such a wide mouth that it is capable "of eating itself" (Figure 22).

In freshwater innumerable other Protists live, such as the Lionotus, a ciliate of considerable size and elegant appearance that using its "proboscis" equipped with a long oral fissure incessantly explores the surrounding environment looking for bacteria and organic detritus. The Trichodina, a ciliate that lives on the epidermis of aquatic animals such as Hydra and the larvae of amphibians and fishes. They resemble little suction cups which have one or two crowns of cilia beneath the cell and one crown above. Thrichodina moves by quickly sliding along the body of the host and they often quarrel with each other. The Stylonychia is a very fast ciliate which knocks everything out of its path. The Lachrymaria has a long and mobile neck. The Chlorella is a small spherical alga that is very common. The Dinoflagellates are also spherical and they also live in the sea.

Where and how to collect Protists

Protists live in stagnant water, so do not search for them in flowing water, water from springs or tap water. Even in the "free" water of a lake or river there are few Protists and so in order to collect them you have to use special nets that concentrate them in a test tube. Instead, look for Protists close to the edge of ponds, puddles, fountains, vases with cut flowers, saucers, rainwater tanks, drains, etc.

Scrape off some of the green patina from a stone or the sides of a fountain. With a knife, scrape some sticks and plants which are immersed in water and collect the material. Collect some algae from a pond. With a pipette draw water from decomposing leaves and other plants.

To collect Protists that live by crawling along, such as amoebae and diatoms, draw water samples near the bottom or on the inclined walls of a pond. When you collect material from the bottom, remember that there is usually a thin layer of decomposing organic material on top of the mud. Avoid taking up the mud because the sand it contains will create problems when you put the sample between the microscope slides. In the anoxic mud, you'll find many bacteria but also some Protists and sand.

With time, a lot of algae gather on the glass walls of an aquarium, causing them to become green. In this layer many mobile organisms love to live, such as Ciliates, Amoebae, Diatoms, Flagellates, etc. Over time, these walls become increasingly opaque and now and then you need to clean them. When you scrape the inside of an aquarium with a special tool equipped with a blade, collect the resulting material in a jar with some water. Similarly, the plastic walls of a garden pond are rich in Protists. Using a pipette, or with the help of a toothbrush, try to detach a little of that patina to suck up.

With a stick, lift up some filamentous algae and collect the water that drains from them in a glass jar. Collect also some filamentous algae because they are very beautiful to look at under the microscope. The green patinas that form in wetlands, on the trunks of trees and on the walls during the cold season can also be interesting to observe.

Rather than mix all of these samples of water in a single container, it is preferable to keep them separate, by placing them into separate pots. Note their provenance on a label.

If you have a garden, make a small pond a few square meters in size. Having some water with Protists near your home is very convenient. In fact, with a pipette you can collect samples of water and place some drops directly onto a slide. For this purpose, it is also sufficient simply to put a white basin into the ground, but during the summer it will tend to dry out and so it must be located in a shady spot. Line the pond with a sheet of white plastic, so you will be better able to see the green patinas that will form. The pond should be at least 30 cm deep to avoid it drying out too quickly in summer. During the autumn, the Protists disappear mysteriously only to return to populate the waters at the end of winter.

Raising protists

If you do not have a body of water such as those described above, you could prepare a culture of Protists. Any container filled with tap water and placed outdoors will soon be filled by microorganisms. For this purpose, leave a bowl of water outside your home in a shaded location. Add a handful of dried grass and within a couple of weeks you will have a lot of microorganisms. However, if you put pond water in the container, it will quickly become populated with a wide variety of protist species. To obtain a culture of Protists a jam jar also works well, but a large container is better e.g. a tank or an aquarium. Once in a while, add some water. If you use tap water, leave it to stand for at least a day to allow the chlorine to evaporate. This gas is added to kill microorganisms and so it could damage your culture.

To achieve a selective culture, you have to collect some Protists of the same species and place them in a container of water enriched with some necessary substances. There are texts that mention the formulas of the culture media for many species of Protists. You can also probably find them on the Internet.

Try to avoid the presence of organisms such as small aquatic crustaceans, mosquito larvae and the larvae of amphibians and tadpoles in the samples of water that you harvest and in your cultures as these feed on protists. If necessary, filter your cultures to remove these small crustaceans and larvae. Cover the containers with a net in order to prevent the deposition of mosquito eggs.


Figure 23 - Deposit of Haematococcus cells
(red colour) and tracks left by the radula of
 a snail which feeds on them.

Raising Haematococcus

Put a couple of litres of water with a handful of earth into a pot and boil for 5 minutes. Let it cool down and decant, then filter the water and throw away the remaining earth. In this way, you get a culture medium enriched with mineral salts. Let the liquid stand for at least a day to allow the atmospheric gases to dissolve in it again.

Now you need to find some Haematococcus. These microscopic algae like to live in small pools of rainwater. When they are in difficulty, they tend to remain on the bottom and become red in colour. It is therefore quite easy to find them because of the clearly visible red stains they form on the bottom and sides of white containers or on the cover. If you do not find them, leave a white plastic bowl outdoors with some clean water or rainwater in it. If you're lucky, after a few days you will see traces of red on the walls and the bottom of the bowl.

Collect some of this reddish deposit and put it into a jar or a tube containing rain water enriched with a little of the culture medium that we described above. In place of rainwater you can also use tap water, provided you let it stand for a day so that any chlorine in it evaporates. Close the tube with a lid that allows air but not dust to enter. If everything works well, after some days and with a strong lens you should note the presence of many Haematococcus. Most likely, there will also be other microorganisms. If you want to purify your culture, place a few drops of it under the stereoscopic microscope and with a pipette or a dropper select some Haematococcus and put them in another tube with water and medium. Now, you should have a reasonably selective culture of Haematococcus.

If you want to plot the numerical growth of your sample, take a drop of the culture and with the stereoscopic microscope count how many Haematococcus there are. Repeat this process every day. Draw a diagram and calculate the time of duplication of the cells.

Every day measure also the pH of the culture. You will notice that it will become increasingly alkaline. In fact, these microscopic Algae remove the carbon dioxide that acidifies the water. Every now and then, add a few drops of carbonated mineral water, which contains a lot of dissolved carbon dioxide, to the culture. The increase in alkalinity will create difficulties for the culture. To remedy this problem, add some drops of vinegar. To keep the pH of the culture steady, add a buffer solution to it.

Some years ago, I tried to make a more challenging culture by building a circuit out of a twenty metre long transparent plastic tube which I rolled up on a table covered with a white plastic sheet. The pipe began and finished at a glass jar with a capacity of a few litres (reactor), in which there was a small water pump for aquaria. Over the table, suspended from the ceiling, I fixed two fluorescent lamps - one white and one red - as I had read that both were suitable for that type of Alga. Unfortunately, I encountered difficulties in preparing the starting culture and in cleaning the tubes. I used bleach, but even after rinsing many times with water, the Haematococcus died. It probably would have been better to use alcohol or salt water. Maybe I'll redo this experiment. This alga is cultivated to obtain astaxanthin, a carotenoid with antioxidant properties.


How to observe Protists

In order to observe Protists you need a microscope for biology. With a pipette or a dropper take some water and organic debris and put some drops on a microscope slide. Mount a coverslip and with a paper towel remove the excess water. With the lowest magnification, explore the slide following a path like that followed by oxen when plowing a field. When you encounter interesting microorganisms, stop and switch to a higher magnification. The most suitable magnification for observing Protists is 250X, but you can also pass to a greater magnification to see the details. The most common technique for observing these microorganisms is the "bright field" but you can get better images in "dark field", in "phase-contrast" and in "Differential Interference Contrast” (DIC). To slow down the movements of Protists during the photo shoots, you can put them in an aqueous solution of gum arabic.

The number of species of Protists is very high, every sample of water has a different population of Protists and you will never cease to be amazed and to discover new ones. Often you will spend hours admiring these tiny organisms.

Normally, the images provided by a microscope are flat and lacking in depth. With the help of polarizing filters you can supply your microscope with the third dimension. To see Protists that move in three-dimensional space is an unforgettable experience. Once you have seen Protists in this manner, you will no longer be able to observe them in any other way [1001].

The reference text to identify and to know the Protists is shown in [103]. It is a very well illustrated atlas with over 1700 drawings of aquatic organisms. It also describes small multicellular freshwater organisms.

If you are interested in science fiction, read this story: "Journey into the Kingdom of Protists" [1002].


In ponds and in different bodies of stagnant water, there are not only protists, but also a variety of multicellular organisms of small or microscopic size. These forms of life are also very interesting to observe. During the observation of protists, you will surely encounter tiny multicellular organisms. Especially in spring, ponds attract insects, amphibians, birds and many other animals, so much so that, as we have already said, they are among the richest environments on the planet in terms of life. The simple study of animal behaviour whilst sitting on the edge of a pond is also useful. Most likely you will see insects that skate on the water, others that shuttle between the surface and the depths and you will see dragonflies that alight on the reeds. You can watch tadpoles and the larvae of newts in the water. As you contemplate the pond life, you will understand which organisms live there and how they interact with each other.

With a bucket, draw some water and some of those piles of green foam-filled algae. By means of a strainer tied to a stick, collect some organisms from the pond that you can see with the naked eye. When you return home, pour that water into an aquarium and examine through the glass walls what you have collected. This sample of water will provide you with an incredible amount of life forms to be studied.

Rotifers (phylum: Rotifera) are multicellular organisms which are slightly larger than the average protist (Figure 24). On the head of some species, there are two crowns of cilia whose quick and synchronised beating makes them resemble wheels in motion. Rotifers are voracious and move incessantly looking for food, living on bacteria, small protozoa and unicellular algae. Sometimes you can see the prey of rotifers in the intestine of the animal, where algae are particularly visible due to their green colour. A distinctive feature of the rotifers is the mastax, a body that crushes the food that it ingests. You will see this organ open and close in the pharynx (throat) of the animal. There are many species of rotifer and they have very different forms. Usually, they have a forked "tail" with which they often temporarily fix themselves to the bottom, or which they use to walk with movements similar to those of a caterpillar.


Figure 24 - Rotifer.

Figure 25 - Hydra (Hydra viridis).

Figure 26 - Gastrotrich.

Sponges, Jellifishes, Hydrae
In clean waters, it is possible to observe sponges (phylum: Porifera) connected to a support and with a porous consistency. jellifishes are very rare in freshwater, but by contrast Hydra are frequently found. Hydrae and jellyfishes both belong to the phylum Cnidaria. Hydrae are polyps and have tentacles. However, you need not be afraid of fighting off a seven-headed Hydra which suddenly emerges from a pond, as these are tiny animals. When Hydrae are contracted, they form a ball of about one millimeter in diameter, and when their tentacles are extended they are 12 millimeters long. In short, they are so small that it is difficult to see them with the naked eye. Hydrae are also usually anchored to the substrate and it is easy to find them attached to the glass walls of aquariums. There are two main types of Hydra, one is beige in colour and the other is green due to the presence of symbiotic algae in its tissues. Hydrae capture small crustaceans with their urticating tentacles and phagocyte them through an opening located amongst the tentacles. Hydrae have a bag-shaped body, they lack an anus and so must expel the waste from the oral opening. Hydrae reproduce by gemmation and it is often possible to observe a daughter Hydra on the body of its mother (Figure 25). Hydrae can move with caterpillar movements or with somersaults.

You can raise sponges in an aquarium. Indeed, from a piece of living sponge it is possible to generate a new colony. This type of activity can allow us to maintain and observe living sponges and analyse their structure. For more information, with a search engine look for the following terms: sponge culture aquarium.

Flatworms and Annelida
In freshwater it is possible to meet many little worms. Among them, we remember the Planaria (phylum: Platyhelmintes, class: Turbellaria), 0.5 - 35 mm long, the Turbellaria and the Microturbellaria (less than 5 mm long). Nematodes (phylum: Nematoda) are thin worms, cylindrical, not segmented, up to 2-3 mm long and that move by wriggling in a characteristic way. It is easy to find these small worms on the bottom of containers of water rich in organic sediments. Also the eelworm is a Nematod and you can find it at the bottom of bottles of vinegar. Gastrotrichs (phylum: Gastrotricha) are small and flat worms very hairy whose body ends with two quite large appendixes. Gstrotrichs are as large as rotifers. While rotifers usually wriggle and only occasionally swim, gastrotrichs cross fast the field of the microscope and in order to observe them you have to do a chase through the movements of the stage that will sorely test your manual skills. Annelida (phylum: Annelida) are worms whose cylindrical body is divided into several segments. Oligochaeta form a subclass of Annelida of different shapes and sizes, many of which have a reddish color due to the presence of hemoglobin in their blood. They are also characterized by the presence of short bristles on the body and most of them moves with quick "8-like" contortions. Usually, they live by swallowing mud and digesting organic particles. Tubifex also are included in this category. The phylum of Annelida includes many families, as leeches and the common earthworms. Other vermiform animals that are commonly found in stagnant water are actually larvae of insects, such as chironomids which, by their small size, the segmentation of the body, color and the rapid contortions can be confused with the oligochaetes.

The first time I saw some bryozoans was while I was cleaning an aquarium in which I was breeding amphibians. I was beginning to clean the inner surface of the glass wall when, with the scraper, I skimmed what appeared to be some branched alga and then saw something contract. Upon closer inspection, I realized that it was a colony of animals which I subsequently identified as Bryozoa. I did not expect to find Bryozoa in freshwater, and especially not in an aquarium. The appearance of a colony of Bryozoa is similar to that of a branched plant, but at the end of each branch lives a zoňid equipped with many tentacles. Through a stereoscopic microscope these animals are extremely beautiful to watch.


Figure 27 - Eggs of freshwater snails.
Notice the eyes and the shell of the embryos

Figure 28 - Embryo of freshwater snail.
Length of the egg = 0.8 mm.

Figure 29 - Tardigrade. L = 0.24 mm.

Water Snails
Species of air-breathing freshwater snails and bivalves (shells) belong to the phylum Mollusca. In ponds, the freshwater snail families Lymnaeidae and Physidae are very common. All species of Lymnaea and Physa lay their eggs in the water inside gelatinous masses about 5 mm wide and 20 mm long. The eggs are distributed within this transparent protective mass and their observation with a microscope is a truly amazing (Figure 27). If you collect one of these masses at the right stage of development, you can see many small eggs in which an already fully-formed embryo moves. Through its transparent shell, you will also be able to see a beating heart. Adult freshwater snails are capable of exploits which are difficult to imagine. In fact, they are able to crawl under the water attaching the foot to the water surface as though it were solid. These snails probably find food floating on the water surface. Freshly-hatched freshwater snails are very pleasant to observe.

Tardigrades, commonly known as water bears, are funny animals that can be considered primitive arthropods. They seem like small transparent bears with 8 little paws each of which terminates in small hooked claws (Figure 29). Despite their delicate appearance, they are capable of withstanding complete dessication for an indefinite period of time, to then resuscitate within an hour or two once they are put back into water. Some species are aquatic, but others prefer moist environments and can be found in moss, lichens, algae, on moist soil, etc. To observe these animals, take some dry moss, crush it and then put it in water. After a few hours you will often find tardigrades.


Figure 30 - Daphnia, a common
water crustacean. L = 1 mm.

Figure 31 - Shrimp of the genus Palaemonetes.

Figure 32 - Gerridae or water-strider / pond skater.

In pond water there are many small crustaceans such as brachiopods (e.g. daphnia), ostracods and copepods. You can collect them with a bucket and if you want you can concentrate them with a strainer. It is interesting to observe them and if you wish to identify them, you have to use a handbook. These animals are in first place on the menu of the larvae of newts, insects and fishes. Another particularly interesting crustacean to raise and observe with the stereoscopic microscope is the shrimp belonging to the genus Palaemonetes (Figure 31). It can be found in brackish water lagoons and even in freshwater lakes and ponds. It is a transparent shrimp that is almost invisible in water. In fact, I noted its presence near the shore of a lake only because of its black eyes that moved in pairs. This shrimp has milky-coloured patterns on its transparent carapace. It lives in brackish water, but can also be bred in freshwater, although it seems incapable of reproducing in this environment. I often found them in lakes used for fishing. You can simply collect these shrimp using a reasonably large strainer mounted on a stick. In an aquarium, you can observe them as they use the small pincers on their legs to collect pieces of food which they then bring to their mouths. If you notice a female with eggs, try putting it in some brackish water in an attempt to hatch the eggs and raise the young.

Aquatic insects
Who said that in order to walk on water you have to be the Messiah? On the surface of a pond, it is easy to see insects that skate and given the ease with which they move, you realize that this is just child's play! If these bugs seem like small dinghies, they could be water-striders / pond skaters (Gerridae) (Figure 32); if they are long and thin, they could be Hydrometra or water measurers; if they have the appearance of small beetles, they could instead be whirligig beetles (Gyrinidae). You can collect these animals with a strainer and put them into a Petri dish to observe them comfortably with a stereoscopic microscope.

Other insects live in water whilst breathing air. Some of them are beetles that have a silver-coloured underside due to a bubble attached to their bodies which is the reserve of oxygen for these tiny divers. Among the aquatic beetles, the water beetle (Dytiscus) is important. In adulthood, this insect has a length of up to 35 mm, and the larval form is a real monster (Figure 35) which is terribly aggressive. Its body is fusiform and it keeps the bottom of the abdomen in contact with the water surface in order to breathe. Its head has two large pointed jaws that the animal keeps open. As soon a tadpole, a fry or a backswimmer passes near the larva of the water beetle, it sinks its fangs into its body. Through its jaws a liquid passes that is injected into the prey to digest its tissues. Sometimes, one of these larvae bites the finger of a person who recklessly passes close to it and this can’t be a pleasant experience.


Figure 33 - Dragonfly nymph. L = 14 mm.

Figure 34 - Backswimmer. L = 12 mm.

Figure 35 - Larva of water beetle.


Of all the insects that live in water, I can’t but mention the backswimmer (Figure 34), an aggressive animal that eats tadpoles and other small prey. The backswimmer is truly incredible as it lives in water but breathes air. So far nothing strange, but this animal, aside from knowing how to swim, is also able to walk and fly. If, with a strainer, you remove some backswimmers from a pond and lay them on the shore, you will soon see them to fly away and then dive again like stones into the stretch of water from which they were taken. So, backswimmers are able to move on land, in air and in water.

There are many insect larvae that live in water. Among the most important are the larvae or nymphs of the dragonfly (Figure 33). These animals really are small monsters. Under their head they have an articulated arm ending in a jaw which can be projected forward to capture prey. If you put pond water containing small crustaceans in an aquarium with dragonflies, you will see these larvae catch crustaceans with motions so rapid that they are difficult to follow - all you will see is the chewing. Often these nymphs are half-buried in the slime. When they swim, they take in water and then expel it from the end of the abdomen, producing a jet that propels them forward. At the end of development, which can last from one to five years depending on the species, the nymphs climb onto a stalk of grass at the edge of the pond where the insect leaves the exuviae, spreads its wings and then flies away. The exuvia remains on the stem and can be gathered for observation under the stereoscopic microscope. Other insect larvae living in water are those of the chironomids, but there are a large number of different species of insects that spend the first part of their life in water. The larva of chironomus is a small segmented Worm that moves with a quick “figure-of-eight”motion. It is often red in colour because of the presence of haemoglobin in the blood. The adult insect is similar to a mosquito but it does not sting.

In Europe, mosquito species are divided principally among three different genera: Culex, Anopheles and Aedes. Culicinae spawn on the water, placing their eggs side by side in a vertical position, forming small rafts of about 1.5 x 5 mm (Figure 36). Anophelinae and Aedes spawn eggs which are isolated from each other. When the eggs hatch, the small larvae of Culex and Aedes position themselves so that the ventral siphon is in contact with the surface in order to breathe. Their body forms an angle of about 60° with the surface of water (Figure 37). The head faces downwards and is equipped with a pair of fan-shaped jaws that beat the water quickly to bring up food particles. On the contrary, the larvae of Anophelinae lie horizontally just under the water surface and the underside of the abdomen has openings that keep them in contact with the atmosphere allowing them to breathe. In comparison with the Culex, Aedes larvae have a shorter and more squat siphon. Figure 38 shows how to use a stereoscopic microscope to observe mosquito larvae and other small aquatic animals.


Figure 36 - Eggs of Culex. The "rafts"
are about 5 mm long and 1.5 mm wide.

Figure 37 - Larvae of Culex.
They are about 6 mm long.

Figure 38 - How to observe and film mosquito
larvae and other small aquatic animals.


When they have grown sufficiently, the mosquito larvae turn into pupae. A few days later, the pupae appear on the surface of the water, the skin of its back tears and a mosquito comes out. Once the mosquito has left the exuviae, the young adult Culex waits several minutes before taking flight, while the young anophele takes flight immediately. These flying syringes now look for warm-blooded animals (birds and mammals) from which to suck the drop of blood they need to produce eggs. The life cycle of mosquitoes usually ranges from 14 to 20 days, thus in a season several generations of mosquitoes are born.

You can find mosquito larvae just below the water surface of a pond or even in small containers. They look like little worms of a few millimetres in length (Figure 37), with a siphon in contact with the surface of the water and with the head facing downwards. By hitting the container, the larvae escape downwards moving with quick contortions. Adult mosquitoes are also interesting to observe. Look at their head, eyes, antennae, the stinging and sucking apparatus, the wings that have scales on the borders which allow them to fly silently and the legs that in the family Aedes have white joints. Usually, on the Aedes mosquito, you can see a white line in the middle of the head and other white streaks on the ventral surface. In order to observe the larvae of mosquitoes you can place them in a small aquarium or in a glass jar (Figure 38).

Mosquitoes are well known as very annoying animals in the adult stage, but few people are able to recognise their larvae. Yet, particularly in regions where there are species capable of spreading disease or very aggressive species such as the tiger mosquito, it is important to recognise these larvae in order to take the necessary steps to eliminate them in time. You can use the microscope to show mosquito larvae to your neighbors, so they learn to recognise them and to ensure that the fight against these annoying insects can be more effective.

Beneath the river pebbles
While you are walking along the banks of a river, in a spot where the water is low and fairly calm, collect some pebbles. On their underside it is easy to find the larvae of insects (Trichoptera, Ephemeroptera, Plecoptera, Megaloptera, etc.). Some of these larvae are enclosed in a box made of grains of sand or small plant fragments. Collect them in a jar with some of the river water. Many of these larvae are equipped with gills located on the underside of the abdomen and they are clearly visible by transparency illumination. On the banks of a river, you can find small wasps that dig their nests in the ground, ichneumonids, as they search for mud to build their jar-shaped nests etc. In the puddles of water near the banks there are small crustaceans and sometimes even fry trapped by the withdrawal of water. The banks of rivers are ecosystems which are rich in small animals that can be observed with a microscope, or a telescope if you want to study their behaviour without disturbing them.

Water Spiders
In a pond, you can see small spiders that run on the surface of the water without sinking, racing forwards and backwards. I never know what they were doing. The Argyroneta aquatica, or water spider, is very interesting. It is a "diver" that builds its home in water but breathes air. It weaves a dense fabric among some aquatic plants, then with a series of trips to the surface, it loads up with air that it releases under the cobweb which then slowly inflates upwards to form a dome. When the air bubble is large enough, the spider settles into it. To look in its lair, you can use a small empty aquarium to be used as a framework.

Eggs and larvae of amphibians
Ponds are the typical ecosystem of many amphibians. Some of them have terrestrial habits, others aquatic. In any case, nearly all amphibians lay their eggs in water and spend the first part of their lives in that element as larvae. The first frogs lay their eggs as soon as ice leaves the stretches of water. This is the time for collecting the newly laid eggs in which the beginning of the segmentation of the eggs is visible. Usually, these newly deposited eggs are very clean and thus suitable for photography. They will subsequently become quickly covered by debris and algae and therefore it is more difficult to observe them.


Figure 39 – Frog eggs at
the fourth segmentation.

Figure 40 - Toad tadpole.

Figure 41 – Newt embryo in the egg.
L = 3 mm. 


The collection of eggs requires knowledge of the suitable time for each species, the location and the appearance of the eggs. Frogs make rounded clusters of hundreds of eggs, toads produce eggs in long strings and newts lay their eggs on the leaves of aquatic plants and to defend them from predators, they fold up the leaf on the egg. To avoid them being carried away by the water, amphibians usually bind their eggs to some plants.

It is also possible to collect the larvae of amphibians. For this purpose, you will need a strainer of approximately 12 centimetres in diameter mounted on a stick. With this tool, you will be able to collect the larvae of species whose eggs you failed to find. For example, it is not easy to find eggs of the tree frog, but it is much easier to collect young tadpoles of this species. In this case also, it is important to recognise the different species of larva.

To look at the development of the eggs and larvae, put them in an aquarium or a small basin containing pond water where the larvae will find protists and small crustaceans on which to feed. In the beginning, these animals do not need you to feed them because they find microorganism in the water in which they live. If it is necessary, feed the tadpoles with boiled spinach and boiled crushed beans. When the tadpoles start to grow legs, and even a few days before, you must provide them with proteins, otherwise the tadpoles of some species tend to eat each other. For this purpose, some flakes of fish food work well. The larvae of newts feed on small crustaceans, small worms and mosquito larvae. When they are close to metamorphosis they like small worms or pieces of earthworm and, in the absence of food, they bites the tails of other newt larvae. These larvae will more readily eat prey that moves, but if they smell fresh pieces of earthworm they will eat them. Give them only a small amount of food, otherwise the water becomes cloudy and they may develop diseases.

Why raise these animals? To save some of them from a drying puddle, to monitor their development and to make observations with the microscope. The newt embryo is contained in a transparent egg and near the end of its development you can observe the heart beating near its throat. In the gills of newt larvae it is possible to observe the flow of the blood cells. This sight, seen under appropriate light, is a true spectacle. On the epidermis of newt, tadpoles and larvae, you can observe more or less branched cells called chromatophores. These are pigmented cells of a variety of colours including metallic gold, silver, brown, orange and black. They have a star-shaped form and therefore they are also called astrocytes. Their shape suggests that they are somehow related to nerve cells. Also on the epidermis of these larvae and other animals such as Fish and Hydrae, you can observe some Trichodina, sucker-shaped protists that move by sliding along the skin of the host. I suggest that you collect a few eggs or larvae and breed them in plenty of clean water. Each week a third of the water must be changed (you have to replace a third of the water). When the amphibians have metamorphosed you have to free them, releasing them in the same place where you collected their eggs.

Do not collect drain water because it can contain dangerous pathogenic microorganisms. As a precaution, when you handle the samples of water, try not to get your hands dirty and wash them occasionally and in particular before eating.

The number of species of small animals that live in freshwater environments is large, hence I have mentioned only the most common. An incomplete, yet much fuller list, is provided by texts such as these: [101] and [102]. These texts are also useful for identifying the different forms of life that you will meet and to have useful information about them.


Along the sea shore, particularly in the coves, reefs, and on the beaches uncovered by the low tide, you can find a lot of beautiful organisms. In particular on the Atlantic coasts such as those of Brittany and Normandy, by looking carefully in puddles of water, you will find small algae, jellyfishes, hermit crabs, sponges, foraminifera and other marine protists, as well as larval forms of many marine animals such as starfishes, sea urchins, crabs and other crustaceans. Normally, these larvae are transparent and you can admire the internal organs. When the fishermen return, you can buy small living animals and eggs to be raised in an aquarium with sea water. You can also get a mask and dive close to shore near the rocks to look for animals that you can then observe under the microscope. If possible, keep this instrument on the beach, in a shaded area provided with a chair, a table and any other necessary tools. Obviously, you will need someone to guard this equipment. Otherwise, you can collect animals in a bucket and observe them at home, always remembering to bring them back to where you collected them. Unfortunately, I do not have enough experience of these environments to give you other suggestions. However, in spite of the shortness of my visits to these environments, it is clear that the marine ecosystems and particularly the coastal ones offer the possibility to make an endless number of wonderful observations.


Foraminifera (Figure 42) are unicellular organisms close to amoebae which live in the sea. They produce snail shaped shells that grow with the organism. It is a limestone shell that, when the animal dies, sinks to the bottom. Throughout geological time, these shells formed large deposits worldwide. Since with time these organisms evolved taking different shapes, their shells are used to date the rocks.

In the sea, you can also collect and observe living foraminifera. On the mainland of any continent, you can find the ground rich in fossil foraminifera. Search for them in lands that in ancient times were submerged by the sea. To observe them it is necessary to separate them from the rock.

Even the beach sand can be interesting to observe. Amid the grains of sand, you can find tiny shells and fragments of marine organisms. Once, I observed sand that came from a Pacific island that was almost entirely composed of the small shells of marine animals, and it was enchanting.

For marine lifeforms, you can refer to the texts [105] e [106].

Figure 42 - Shells of fossil Foraminifers.


Children like to do strange experiments, even though they know that they will not really work. For example, with some cardboard boxes it is possible to build a spaceship and have an eventful interplanetary journey. Ever since I was a boy, I have liked microscopes. I remember when I tried to lengthen the tube of my microscope to see what would happen. It was a home-made microscope, although the objective and the eyepiece were bought and of good quality. I had taken a tube of about a metre long which I placed on the microscope, and in order to look through the eyepiece I was forced to stand on a chair. At the beginning, I could not see anything, but after a while I got accustomed to the situation and I managed to see some highly magnified protozoa. Moving the eyepiece, I could explore the area even though the light was low. While trying to follow an alga that was swimming quickly, the weight of the pipe caused it to fall on the slide breaking it and leading to a catastrophic event in that miniature world.

Another interesting experiment can be carried out with pond water containing protozoa. This experiment involves soaking up water from one side of the preparation and adding distilled water to the other side. Soon you will see the protists swell and swell until they explode. You will see their organelles disperse in the water and you will see the cilia of the "mouth" of the microorganism continue to flap as if nothing had happened. You can conduct a similar experiment, this time by adding slightly soapy water. The surfactant molecules of which the soap and detergents are composed attack the molecules of the protists membrane making wide openings. The membrane of the nucleus will be demolished, freeing the DNA. It is for this reason that we must avoid dumping so much detergent in the environment.

As we have seen, water-striders / pond skaters like to skate on the surface of ponds and they do this by exploiting the surface tension of the water and the water-repellent properties of their feet. With a drop of detergent, it is possible to reduce the surface tension of the water and put these skaters in serious difficulty. This test highlights the importance, at least for these insects, of the properties of water. With a strainer, collect a Water-strider and put it into a bowl containing some water. If you add one little drop of detergent to the water, the water-strider will not be able to remain on the surface for long and will sink. Pick up it immediately because otherwise it will drown. Do not repeat the experiment with the same animal. If you conduct the same experiment on mosquito larvae, you should notice the difficulty of the larvae in maintaining their siphon in stable contact with the surface of the water.


As you explore with the microscope, you should study the following topics: the origin of life on Earth, the theory of evolution, the theory of natural selection, the cell (shape and organisation), genes and chromosomes, the karyotype, cell division (mitosis), fertilisation and meiosis, DNA and the synthesis of proteins, genes and inheritance, systematics and the classification of organisms, the difference between viruses, prokaryotes and eukaryotes, the Protists and multicellular organisms, the different divisions of the Phylum Protista and of the small multicellular organisms.

To deepen your understanding of Protists, you can read the first 78 pages of the text listed in [103]. This text will serve primarily as an atlas for the recognition of protists and some small aquatic organisms.
The text shown in [101] is more suitable for increasing your knowledge of aquatic multicellular animals. In this text, study at least the features of the different main groups. You can read the information regarding families, genera and species as you come across them.
Text [102] is suitable as an atlas.
The tables that deal with algae [003] are very interesting.
The text indicated in [106] is useful for those who want to examine the various organisms that they meet from an evolutionary perspective. This is a university textbook and so is quite challenging, but it has the great advantage that it is accompanied by useful drawings.


Aquatic ecosystems are very rich in lifeforms. The observation of small organisms living in these environments does not require any special preparations, such as are necessary for histology, so it is very simple and immediate. Moreover, it is a source of continual wonder. For a child, watching the variety of forms and observing the behaviour of these organisms, in addition to being fun, is very educational and helps them to open their mind. The observation of freshwater organisms can be limited to the mere contemplation of their forms and behaviour, but it can also be done for school or to expand the knowledge of nature, by placing an emphasis on the biological structures conquered by the various living beings during their evolution.



R Fitter and R Manuel; Collins field guide to freshwater life; Collins, 1986, 759 drawings, 332 pictures. 

A guide to the fauna and flora of the European freshwater systems.


H. Bellmann; Leben in Bach und Teich (Life in streams and ponds); Mosaik Munchen, 1988

Hundreds of drawings, pictures and information regarding freshwater organisms.


H. Streble, D. Krauter; Das Leben im Wassertropfen (The Life in a drop of water); Frankh'sche Verlagshandlung, Keller, Stuttgart, 1981.

An atlas with more than 1700 drawings that are very useful for identifying protists. This is a valuable collection of drawings and information on freshwater unicellular organisms. The first part is a guide to the collection, the culture and the observation of protists.




V., J. Pearse, M. Buchsbaum, R. Buchsbaum; Living Invertebrates; Pacific Grove, California: The Boxwood Press, 1987.

A useful text for those who are interested in marine invertebrates.


M. La Greca; Zoologia degli Invertebrati (Zoology of Invertebrates); Utet

A university text which is very useful for those who want in-depth knowledge of the structure of the invertebrates from an evolutionary perspective. It also deals with the Protists. The drawings are very well done and very useful.


Barnes R.D. Invertebrate Zoology . (V ed.), 1991, Sanders HBJ.

See also the texts and Internet resources of a general character which are indicated in the presentation article of this guide.


1001 -  How to enable a microscope for high magnification stereoscopy.
1002 -  A Journey in the Kingdom of Protists (a novel).
1003 -  Protist Park. The first park in the world devoted to the protists (with movies).
1004 -  Movie with bacteria (small) and a blue-green alga (large and green in colour).
1005 -  Photomicrography Charles Krebs, beautiful images of diatoms.
1006 -  Information on microscopy and wonderful images of diatoms by Alessandro Bertoglio.
1007 -  Pictures of diatoms by A. Bertoglio from the slide of 100 forms of Klaus D. Kemp.
1008 -  Microscopy society: "Amici del Microscopio".
1009 -  A user-friendly guide to coastal planktonic ciliates.
1010 -  Protist Information Server.
1011 -  Protist-related websites - professional resources.
1012 -  Prokaryotes, Eukaryotes, & Viruses Tutorial.
1013 -  Protozoa - Some common freshwater types.
1014 -  Pond Life Movie Gallery.
1015 - 
1016 -
1017 -  Micrographia

Internet keywords: protists, protozoa, unicellular algae, freshwater algae.

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