Freshwater hydra breathing. Hydra freshwater polyp. The body shape of hydra is tubular. The mouth opening of these animals is covered with tentacles. Hydras live in water, and with their stinging tentacles they kill and bring prey to their mouths.

Hydra is a genus of animals belonging to the Coelenterates. Their structure and activity are often considered on the example of a typical representative - freshwater hydra. Further, this particular species will be described, which lives in fresh water bodies with clean water attached to aquatic plants.

Usually the size of the hydra is less than 1 cm. life form- a polyp, which suggests a cylindrical shape of the body with a sole at the bottom and a mouth opening on the upper side. The mouth is surrounded by tentacles (approximately 6-10), which can be extended in length exceeding the length of the body. The hydra leans in the water from side to side and with its tentacles catches small arthropods (daphnia, etc.), after which it sends them into the mouth.

For hydras, as well as for all coelenterates, it is characteristic radial (or radial) symmetry. If you look at not from above, then you can draw a lot of imaginary planes dividing the animal into two equal parts. Hydra does not care which side food swims up to it, since it leads a motionless lifestyle, therefore, radial symmetry is more beneficial for it than bilateral symmetry (characteristic of most mobile animals).

Hydra's mouth opens into intestinal cavity. This is where the digestion of food takes place. The rest of digestion is carried out in cells that absorb partially digested food from the intestinal cavity. Undigested residues are ejected through the mouth, since coelenterates do not have an anus.

The body of the hydra, like all coelenterates, consists of two layers of cells. outer layer called ectoderm, and the inner endoderm. Between them there is a small layer mesoglea- non-cellular gelatinous substance, which may contain various types of cells or processes of cells.

Hydra ectoderm

Hydra ectoderm is made up of several types of cells.

skin muscle cells the most numerous. They create the integuments of the animal, and are also responsible for changing the shape of the body (elongation or reduction, bending). Their processes contain muscle fibers that can contract (while their length decreases) and relax (their length increases). Thus, these cells play the role of not only covers, but also muscles. Hydra does not have real muscle cells and, accordingly, real muscle tissue.

The Hydra can move around using somersaults. She leans so hard that she reaches the support with her tentacles and stands on them, lifting the sole up. After that, the sole already leans and becomes on a support. Thus, the hydra makes a somersault and finds itself in a new place.

The hydra has nerve cells. These cells have a body and long processes that connect them to each other. Other processes are in contact with skin-muscle and some other cells. Thus, the whole body is enclosed in a nervous network. Hydra does not have an accumulation of nerve cells (ganglia, brain), however, even such a primitive nervous system allows them to have unconditioned reflexes. Hydras respond to touch, the presence of a number chemical substances, temperature change. So if you touch the hydra, it shrinks. This means that excitation from one nerve cell spreads to all the others, after which the nerve cells transmit a signal to the skin-muscle cells so that they begin to contract their muscle fibers.

Between the skin-muscle cells, the hydra has a lot of stinging cells. Especially a lot of them on the tentacles. These cells inside contain stinging capsules with stinging filaments. Outside, the cells have a sensitive hair, when touched, the stinging thread shoots out of its capsule and strikes the victim. In this case, poison is injected into a small animal, usually having a paralytic effect. With the help of stinging cells, the hydra not only catches its prey, but also defends itself from animals attacking it.

intermediate cells(located in the mesoglea rather than in the ectoderm) provide regeneration. If the hydra is damaged, then, thanks to the intermediate cells, new various cells of the ectoderm and endoderm are formed at the site of the wound. The Hydra can regenerate a fairly large portion of its body. Hence its name: in honor of the character of ancient Greek mythology, who grew new heads to replace the severed ones.

Hydra endoderm

The endoderm lines the intestinal cavity of the hydra. The main function of endoderm cells is to capture food particles (partially digested in the intestinal cavity) and their final digestion. At the same time, endoderm cells also have muscle fibers that can contract. These fibrils are directed towards the mesoglea. Flagella are directed towards the intestinal cavity, which scoop up food particles to the cell. The cell captures them the way amoeba do - forming pseudopods. Further, the food is in the digestive vacuoles.

The endoderm secretes a secret into the intestinal cavity - digestive juice. Thanks to him, the animal captured by the hydra breaks up into small particles.

Hydra breeding

The freshwater hydra has both sexual and asexual reproduction.

asexual reproduction carried out by budding. It occurs during a favorable period of the year (mainly in summer). A protrusion of the wall forms on the body of the hydra. This protrusion increases in size, after which tentacles form on it and a mouth erupts. Subsequently, the daughter individual is separated. Thus, freshwater hydras do not form colonies.

With the onset of cold weather (in autumn), the hydra transgresses to sexual reproduction. After sexual reproduction, hydras die, they cannot live in winter. During sexual reproduction in the body of the hydra, eggs and sperm are formed. The latter leave the body of one hydra, swim up to another and fertilize her eggs there. Zygotes are formed, which are covered with a dense shell that allows them to survive the winter. In the spring, the zygote begins to divide, and two germ layers are formed - the ectoderm and endoderm. When the temperature gets high enough, the young hydra breaks the shell and comes out.

The body shape of hydra is tubular. The mouth opening of these animals is covered with tentacles. Hydras live in water, and with their stinging tentacles they kill and bring prey to their mouths.

   Basic data:
DIMENSIONS
Length: 6-15 mm.

BREEDING
Vegetative: has a budding character. A kidney appears on the body of the mother individual, from which the daughter individual gradually develops.
Sexual: most types of hydras have separate sexes. The gonads accumulate cells from which eggs develop. Sperm develop in the testis.

LIFESTYLE
Habits: live in fresh and brackish waters.
Food: plankton, fish fry, ciliates.
Lifespan: no data.

RELATED SPECIES
More than 9,000 species belong to the type of coelenterates, some of them (15-20) live only in fresh waters.

   Freshwater hydras are one of the smallest predators. Despite this, they are able to provide themselves with food. Hydras have a tubular body shape. With the help of the sole, they attach themselves to underwater plants or rocks and move their tentacles in search of prey. Green hydras contain photosynthetic algae.

FOOD

LIFESTYLE

• The Hydra does not age, as every cell in its body is renewed after a few weeks. This animal lives only in the warm season. With the onset of winter, all adult hydras die. Only their eggs, protected by a strong double shell, the embryotheca, can overwinter.
• Hydras easily regenerate their lost limbs. The scientist G. Tremblay (1710-1784), as a result of his numerous experiments, received a seven-headed polyp, in which severed heads grew. He looked like a mythical creature - the Lernean Hydra, defeated by the hero of ancient Greece - Hercules.
• During constant movements in the water, the hydra produces quite original acrobatic stunts.

CHARACTERISTIC FEATURES OF THE HYDRA

Hydra is a typical representative of the Hydrozoa class. It has a cylindrical body shape, reaching a length of up to 1-2 cm. At one pole there is a mouth surrounded by tentacles, the number of which in different species varies from 6 to 12. At the opposite pole, the hydra has a sole that serves to attach the animal to the substrate.

sense organs

In the ectoderm, hydras have stinging or nettle cells that serve to protect or attack. In the inner part of the cell is a capsule with a spiral thread.

Outside this cell is a sensitive hair. If any small animal touches a hair, then the stinging thread rapidly shoots out and pierces the victim, who dies from the poison that has fallen along the thread. Usually many stinging cells are ejected simultaneously. Fish and other animals do not eat hydras.

Tentacles serve not only for touch, but also for capturing food - various small aquatic animals.

In the ectoderm and endoderm, hydras have epithelial-muscular cells. Thanks to the contraction of the muscle fibers of these cells, the hydra moves, “stepping” alternately either with tentacles or with the sole.

Nervous system

The nerve cells that form a network throughout the body are located in the mesoglea, and the processes of the cells extend outside and inside the body of the hydra. This type of structure of the nervous system is called diffuse. Especially a lot of nerve cells are located in the hydra around the mouth, on the tentacles and soles. Thus, the simplest coordination of functions already appears in the coelenterates.

Hydrozoans are irritable. When nerve cells are irritated by various stimuli (mechanical, chemical, etc.), the perceived irritation spreads to all cells. Due to the contraction of muscle fibers, the body of the hydra can be compressed into a ball.

Thus, for the first time in organic world coelenterates have reflexes. In animals of this type, the reflexes are still uniform. In more organized animals, they become more complex in the process of evolution.

Digestive system

All hydras are predators. Having captured, paralyzed and killed the prey with the help of stinging cells, the hydra pulls it with its tentacles to the mouth opening, which can stretch very strongly. Further, the food enters the gastric cavity, lined with glandular and epithelial-muscular cells of the endoderm.

Digestive juice is produced by glandular cells. It contains proteolytic enzymes that promote protein digestion. Food in the gastric cavity is digested by digestive juices and breaks down into small particles. In the cells of the endoderm, there are 2-5 flagella that mix food in the gastric cavity.

Pseudopodia of epithelial-muscular cells capture food particles and further intracellular digestion occurs. Undigested food remains are removed through the mouth. Thus, in hydroids, for the first time, cavitary, or extracellular, digestion appears, running in parallel with more primitive intracellular digestion.

Organ regeneration

In the ectoderm, the hydra has intermediate cells, from which, when the body is damaged, nerve, epithelial-muscular and other cells are formed. This contributes to the rapid overgrowth of the wounded area and regeneration.

If a Hydra's tentacle is cut off, it will regenerate. Moreover, if the hydra is cut into several parts (even up to 200), each of them will restore the whole organism. On the example of hydra and other animals, scientists are studying the phenomenon of regeneration. The revealed patterns are necessary for the development of methods for treating wounds in humans and many vertebrate species.

Hydra breeding methods

All hydrozoans reproduce in two ways - asexual and sexual. Asexual reproduction is as follows. In the summer, approximately in the middle, the ectoderm and endoderm protrude from the body of the hydra. A tubercle, or kidney, is formed. Due to the multiplication of cells, the size of the kidney increases.

The gastric cavity of the daughter hydra communicates with the cavity of the mother. A new mouth and tentacles form at the free end of the kidney. At the base, the kidney is laced, the young hydra is separated from the mother and begins to lead an independent existence.

Sexual reproduction in hydrozoans under natural conditions is observed in autumn. Some types of hydras are dioecious, while others are hermaphroditic. In freshwater hydra, female and male sex glands, or gonads, are formed from the intermediate cells of the ectoderm, that is, these animals are hermaphrodites. The testicles develop closer to the oral part of the hydra, and the ovaries develop closer to the sole. If many motile spermatozoons are formed in the testes, then only one egg matures in the ovaries.

Hermaphroditic individuals

In all hermaphroditic forms of hydrozoans, spermatozoons mature earlier than eggs. Therefore, fertilization occurs crosswise, and consequently, self-fertilization cannot occur. The fertilization of the eggs occurs in the mother as early as autumn time. After fertilization, the hydra, as a rule, die, and the eggs remain in a dormant state until spring, when new young hydra develop from them.

budding

Marine hydroid polyps can be solitary like hydras, but more often they live in colonies that have appeared due to the budding of a large number of polyps. Polyp colonies often consist of a huge number of individuals.

In marine hydroid polyps, in addition to asexual individuals, during reproduction by budding, sexual individuals, or jellyfish, are formed.

The naturalist A. Leeuwenhoek, who invented the microscope, was the first to be able to see and describe the hydra. This scientist was the most significant naturalist of the XVII-XVIII centuries.

Examining aquatic plants with his primitive microscope, Leeuwenhoek noticed a strange creature that had hands "in the form of horns." The scientist even observed the budding of these creatures and saw their stinging cells.

The structure of freshwater hydra

Hydra refers to intestinal animals. Its body has a tubular shape, in front there is a mouth opening, which is surrounded by a corolla, consisting of 5-12 tentacles.

Under the tentacles, the body of the hydra narrows and a neck is obtained, which separates the body from the head. The back of the body is narrowed into a stalk or stalk, with a sole at the end. When the hydra is full, its body does not exceed 8 millimeters in length, and if the hydra is hungry, the body is much longer.

Like all representatives of the intestinal cavity, the body of the hydra is formed by two layers of cells.

The outer layer consists of a variety of cells: some cells are used to defeat prey, other cells have contractility, and still others secrete mucus. And in the outer layer there are nerve cells that form a network that covers the body of the guides.

Hydra is one of the few representatives of the coelenterates that lives in fresh water, and most of these creatures live in the seas. The habitat of hydras is a variety of water bodies: lakes, ponds, ditches, river backwaters. They settle on aquatic plants and roots of duckweed, which covers the entire bottom of the reservoir with a carpet. If the water is clean and transparent, then the hydras settle on the stones near the shore, sometimes forming a velvet carpet. Hydras love light, so they prefer shallow places near the coast. These creatures can discern the direction of light and move towards its source. If hydras live in an aquarium, they always move to its illuminated part.

If aquatic plants are placed in a vessel with water, then you can see how hydras crawl along their leaves and walls of the vessel. On the sole of the hydra there is an adhesive substance that helps it to firmly attach to aquatic plants, stones and the walls of the aquarium, it is quite difficult to tear the hydra from its place. Occasionally, the hydra moves in search of food, this can be observed in aquariums when a trace remains on the stack in the place where the hydra sat. In a few days, these creatures move no more than 2-3 centimeters. During movement, the hydra is attached to the glass with a tentacle, tears off the sole and drags it to a new place. When the sole is attached to the surface, the hydra levels off and rests on its tentacles again, taking a step forward.

This method of movement is similar to the movement of moth caterpillars, which are often called "surveyors". But the caterpillar pulls the rear to the front and then moves the front again. And the hydra flips over its head every time it moves. So the hydra moves fast enough, but there is another, slower way to move - when the hydra slides on its sole. Some individuals can detach from the substrate and swim in the water. They spread their tentacles and sink to the bottom. And hydras rise up with the help of a gas bubble that forms on the sole.

How do freshwater hydras eat?

Hydras are predatory creatures, they feed on ciliates, cyclops, small crustaceans- daphnia and other small animals. Sometimes they eat larger prey, such as small worms or mosquito larvae. Hydras can even wreak havoc on fish ponds as they feed on newly hatched fish.

How the hydra hunts can be easily traced in the aquarium. She spreads her tentacles widely, which form a web, while she hangs tentacles down. If you watch the hydra, you will notice that its body, slowly swaying, describes a circle with its front part. A passing victim catches on the tentacles, tries to free itself, but calms down as the stinging cells paralyze it. Hydra pulls prey to the mouth and begins to eat.

If the hunt is successful, the hydra swells from the number of crustaceans eaten, and their eyes appear through its body. Hydra can eat prey larger than itself. The mouth of the hydra is able to open wide, and the body is significantly stretched. Sometimes a part of the victim sticks out of the mouth of the hydra, which did not fit inside.

Reproduction of freshwater hydra

If there is enough food, hydras multiply rapidly. Reproduction occurs by budding. The process of kidney growth from a tiny tubercle to a mature individual takes several days. Often, several buds are formed on the body of the hydra, while the young individual has not separated from the mother hydra. Thus, asexual reproduction occurs in hydras.

In autumn, when the water temperature drops, hydras can also reproduce sexually. On the body of the hydra, the sex glands are formed in the form of swellings. In some swellings, male sex cells are formed, and in others, egg cells. Male sex cells float freely in the water and penetrate into the hydra body cavity, fertilizing immobile eggs. When eggs are formed, the hydra usually dies. Under favorable conditions, young individuals emerge from the eggs.

Freshwater hydra regeneration

Hydras have an amazing ability to regenerate. If the hydra is cut in half, then new tentacles will quickly grow in the lower part, and the sole on the upper part.

In the 17th century, the Dutch scientist Tremblay spent with hydras interesting experiments, as a result of which he not only managed to grow new hydras from pieces, but also spliced ​​different halves of hydras, getting seven-headed polyps and turning their bodies inside out. When a seven-headed polyp was obtained, similar to a hydra from Ancient Greece, these polyps became known as hydras.

Hydra biology description internal structure photo lifestyle nutrition reproduction protection from enemies

Latin name Hydrida

To characterize the structure of a hydroid polyp, one can use as an example freshwater hydras, which retain very primitive features of organization.

External and internal structure

Hydra have an elongated, sac-like body that can stretch quite strongly and shrink almost into a spherical lump. A mouth is placed at one end; this end is called the mouth or oral pole. The mouth is located on a small elevation - the oral cone, surrounded by tentacles that can stretch and shorten very strongly. In the extended state, the tentacles are several times the length of the hydra's body. The number of tentacles is different: they can be from 5 to 8, and some hydras have more. In hydra, a central gastric, somewhat more expanded section is distinguished, turning into a narrowed stalk ending in a sole. With the help of the sole, the hydra is attached to the stems and leaves. aquatic plants. The sole is located at the end of the body, which is called the aboral pole (opposite to the mouth, or oral).

The wall of the body of the hydra consists of two layers of cells - ectoderm and endoderm, separated by a thin basal membrane, and limits the only cavity - the gastric cavity, which opens outward with a mouth opening.

In hydras and other hydroids, the ectoderm is in contact with the endoderm along the very edge of the mouth opening. In freshwater hydras, the gastric cavity continues into hollow tentacles inside, and their walls are also formed by ectoderm and endoderm.

The ectoderm and endoderm of the hydra are composed of a large number of cells various types. The main mass of cells of both the ectoderm and endoderm are epithelial-muscular cells. Their outdoor cylindrical part similar to ordinary epithelial cells, and the base, adjacent to the basal membrane, is elongated spindle-shaped and represents two contractile muscular processes. In the ectoderm, the contractile muscular processes of these cells are elongated in the direction of the longitudinal axis of the hydra body. Their contractions cause shortening of the body and tentacles. In the endoderm, the muscular processes are elongated in an annular direction, across the axis of the body. Their contraction has the opposite effect: the body of the hydra and its tentacles narrow and lengthen at the same time. Thus, the muscle fibers of the epithelial-muscular cells of the ectoderm and endoderm, opposite in their action, make up the entire musculature of the hydra.

Among the epithelial-muscular cells, various stinging cells are located either singly or, more often, in groups. The same type of hydra, as a rule, has several types of stinging cells that perform different functions.

The most interesting are stinging cells with nettle properties, called penetrants. These cells throw out a long thread when stimulated, which pierces the body of the prey. The stinging cells are usually pear-shaped. A stinging capsule is placed inside the cell, covered with a lid on top. The wall of the capsule continues inward, forming a neck, which passes further into a hollow thread, coiled into a spiral and closed at the end. At the point of transition of the neck into the thread, there are three spines inside, folded together and forming a stylet. In addition, the neck and stinging thread are seated inside with small spines. On the surface of the stinging cell there is a special sensitive hair - knidocil, with the slightest irritation of which the stinging thread is ejected. First, the lid opens, the neck is twisted, and the stylet pierces into the cover of the victim, and the spikes that make up the stylet move apart and widen the hole. Through this hole, the eversible thread pierces the body. Inside the stinging capsule contains substances that have nettle properties and paralyze or kill prey. Once fired, a stinging thread cannot be used again by a hydroid. Such cells usually die and are replaced by new ones.

Another kind of stinging cells of hydra are volvents. They do not have nettle properties, and the threads they throw out serve to hold prey. They wrap around the hairs and bristles of crustaceans, etc. The third group of stinging cells are glutinants. They throw out sticky threads. These cells are important both in holding prey and in moving the hydra. The stinging cells are usually, especially on the tentacles, arranged in groups - "batteries".

In the ectoderm there are small undifferentiated cells, the so-called interstitial cells, due to which many types of cells develop, mainly stinging and sex cells. Interstitial cells are often located in groups at the base of epithelial-muscular cells.

The perception of stimuli in hydra is associated with the presence in the ectoderm of sensitive cells that serve as receptors. These are narrow, tall cells with a hair on the outside. Deeper, in the ectoderm, closer to the base of the skin-muscle cells, there are nerve cells equipped with processes, with the help of which they contact each other, as well as with receptor cells and contractile fibers of the skin-muscle cells. Nerve cells are scattered in the depths of the ectoderm, forming with their processes a plexus in the form of a mesh, and this plexus is denser on the perioral cone, at the base of the tentacles and on the sole.

The ectoderm also contains glandular cells that secrete adhesive substances. They are concentrated on the sole and on the tentacles, helping the hydra to temporarily attach to the substrate.

Thus, in the hydra ectoderm there are cells of the following types: epithelial-muscular, stinging, interstitial, nervous, sensitive, glandular.

The endoderm has less differentiation of cellular elements. If the main functions of the ectoderm are protective and motor, then the main function of the endoderm is digestive. In accordance with this, most of the endoderm cells consist of epithelial-muscular cells. These cells are equipped with 2-5 flagella (usually two), and are also able to form pseudopodia on the surface, capture them, and then digest food particles. In addition to these cells, the endoderm contains special glandular cells that secrete digestive enzymes. In the endoderm there are also nerve and sensory cells, but in much smaller numbers than in the ectoderm.

Thus, several types of cells are also represented in the endoderm: epithelial-muscular, glandular, nervous, and sensitive.

Hydras do not always remain attached to the substrate, they can move from one place to another in a very peculiar way. Most often, hydras move “walking”, like caterpillars of moth butterflies: the hydra tilts its oral pole to the object on which it sits, sticks to it with tentacles, then the sole breaks off from the substrate, pulls up to the oral end and attaches again. Sometimes the hydra, having attached its tentacles to the substrate, raises the stem with the sole up and immediately brings it to the opposite side, as if “tumbling”.

Hydra Power

Hydras are predators, they sometimes feed on rather large prey: crustaceans, insect larvae, worms, etc. With the help of stinging cells, they capture, paralyze and kill prey. Then the victim is pulled by tentacles to a highly extensible mouth opening and moves into the gastric cavity. In this case, the gastric part of the body swells strongly.

Digestion of food in hydra, unlike sponges, only partially occurs intracellularly. This is due to the transition to predation and the capture of rather large prey. The secret of the glandular cells of the endoderm is secreted into the gastric cavity, under the influence of which the food softens and turns into gruel. Small food particles are then captured by the digestive cells of the endoderm, and the digestion process is completed intracellularly. Thus, for the first time in hydroids, intracellular or cavitary digestion occurs, which occurs simultaneously with more primitive intracellular digestion.

Protection from enemies

Hydra nettle cells not only infect prey, but also protect the hydra from enemies, causing burns to predators attacking it. And yet there are animals that feed on hydras. Such are, for example, some ciliary worms, and especially Microstomum lineare, some gastropods(pond snails), Corethra mosquito larvae, etc.

Hydra's ability to regenerate is very high. Experiments conducted by Tremblay as early as 1740 showed that pieces of the hydra's body, cut into several dozen pieces, regenerate into a whole hydra. However, a high regenerative capacity is characteristic not only of hydras, but also of many other intestinal cavities.

reproduction

Hydras reproduce in two ways - asexual and sexual.

Asexual reproduction of hydras occurs by budding. IN natural conditions hydra budding occurs throughout summer period. Under laboratory conditions, hydra budding is observed with fairly intensive nutrition and a temperature of 16-20 ° C. Small swellings form on the body of the hydra - buds, which are a protrusion of the ectoderm and endoderm. In them, due to multiplying cells, further growth of the ectoderm and endoderm occurs. The kidney increases in size, its cavity communicates with the gastric cavity of the mother. At the free, outer end of the kidney, tentacles and a mouth opening finally form.

Soon, the formed young hydra is separated from the mother.

Sexual reproduction of hydras in nature is usually observed in autumn, and in laboratory conditions it can be observed with malnutrition and temperatures below 15-16 ° C. Some hydras are dioecious (Relmatohydra oligactis), others are hermaphrodites (Chlorohydra viridissima).

Sex glands - gonads - arise in hydra in the form of tubercles in the ectoderm. In hermaphroditic forms, male and female gonads are formed in different places. The testes develop closer to the oral pole, while the ovaries develop closer to the aboral. Formed in the testicles a large number of motile sperm. Only one egg matures in the female gonad. In hermaphroditic forms, the maturation of spermatozoa precedes the maturation of eggs, which ensures cross-fertilization and excludes the possibility of self-fertilization. The eggs are fertilized in the body of the mother. A fertilized egg puts on a shell and hibernates in this state. Hydras, after the development of reproductive products, as a rule, die, and in the spring a new generation of hydras comes out of the eggs.

Thus, in freshwater hydras, under natural conditions, a seasonal change in the forms of reproduction occurs: throughout the summer, hydras intensively bud, and in autumn (for middle lane Russia - in the second half of August), with a decrease in temperature in water bodies and a decrease in the amount of food, they cease to reproduce by budding and proceed to sexual reproduction. In winter, hydras die, and only fertilized eggs overwinter, from which young hydras emerge in spring.

The hydra also includes the freshwater polyp Polypodium hydriforme. early stages the development of this polyp takes place in the eggs of sterlets and causes them great harm. Several types of hydra are found in our reservoirs: stalked hydra (Pelmatohydra oligactis), common hydra(Hydra vulgaris), green hydra (Chlorohydra viridissima) and some others.