UNICELLULAR animals such as Amoeba and Paramecium belong to the group Protozoa, but all multicellular animals, I.e. animals which are built up of many cells, are placed in the group Metazoa. Among the simplest of the Metazoa are the animals called the Ccelenterates, and Hydra may be studied as an example of this class. The name Ccelenterate means ’hollow inside, ’ for the animals belonging to this class have only one cavity within the body, which communicates with the exterior only through the mouth. This cavity is called the enteron. Hydra, or the Fresh Water Polyp as it is sometimes called, is to be found in ponds and streams, and is generally attached to the leaf or stem of a water plant. The body is cylindrical in shape and is fixed by the base or foot to the support, while at the unattached end it bears a ring of hollow thread-like tentacles. These surround a small prominence called the hypostome, at the top of which is the mouth. When fully extended a mature Hydra may reach a length of nearly half an inch, but when irritated the polyp contracts into a short blob on which the tentacles are only visible as tiny bulges. Two or three rounded knobs may be present on the outside of the body of Hydra, between the foot and the tentacles. These are the reproductive organs, the male organ or testis being situated near the tentacle end and the female organ or ovary near the foot. Frequently a small Hydra bud, shaped like a miniature adult, is to be seen growing out from the region of the body near the foot.

Three species of Hydra are found in Great Britain, and are recognizable by their colour. Hydra viridis is green, Hydra fusca is brown, while Hydra vulgaris is practically colourless.

The Internal Structure of Hydra

The body wall of Hydra consists of two layers of cells , an outer layer called the ectoderm and an inner one called the endoderm. The two layers are separated by a sheet of gelatinous, non-cellular material, the mesoglcea, which is secreted by the cells.

The Ectoderm

There are several kinds of ectoderm cells.

The Musculo-epithelial Cells

These are the most numerous and are large pear-shaped cells with broad outer ends covered by a thin membrane which forms a continuous protective surface for the body. Each cell has a large nucleus lying in the central cytoplasm, and from the base of the cell one or more narrow protoplasmic extensions, called muscle-tails, project. The muscle-tails are embedded in the mesoglcea and are arranged so that they lie parallel to the column of the body. By the contraction of these muscle-tails the body of Hydra becomes shorter and fatter. Certain cells in the ectoderm of the foot produce a secretion for anchoring the part of the animal to a suitable substratum.

The Interstitial Cells

Occurring between the bases of the musculo-epithelial cells are small rounded cells termed the interstitial cells. These cells are able to develop into and replace any cells of the body which have become lost owing to injury. The increase in the number of interstitial cells at certain places on the column causes the formation of swellings which are the reproductive organs. Buds are also caused by the multiplication of the interstitial cells. The Cnidoblast

This is a pear-shaped ectoderm cell in which lies a large sac, the nematocyst, containing liquid. At the outer end the sac is produced into a long thread, which is tucked into the inside of the sac in the same way as the end of an empty glove finger which has been pushed into the glove. The thread lies inside the nematocyst, coiled in a spiral. Projecting from the outer side of the cnidoblast is a small pointed process, the cnidocil or trigger hair. If the cnidocil is touched or affected by substances in solution in the water, the protoplasm of the cnidoblast contracts so as to compress the nematocyst and cause the coiled thread to be turned inside out and discharged from the sac. There are three kinds of ncmatocysts , of which the largest have barbs at the base of the thread and are used for offence and defence. If a small water animal strikes the cnidocil and so causes the thread to be ejected, the barbs wound the animal and the thread enters its body. Then a poisonous fluid passes down the thread and paralyses the body of the victim. The second type of nematocyst has no barbs, and has a thread which becomes spirally twisted when discharged. The spiral thread holds the prey by becoming entangled in the bristles of its body. The third type of nematocyst has a straight, barbless thread which is sticky, and so also helps to prevent the escape of the prey.

The cnidoblasts are found on the column of the body but are most numerous on the tentacles, where they occur in groups or batteries, which appear as wart-like prominences. Each battery consists of two or three large nematocysts surrounded by several of the smaller type. After use a cnidoblast is shed and replaced by a new one, formed from one of the interstitial cells.

Nerve Cells

Small nerve cells occur in the ectoderm between the bases of the musculo-epithelial cells. Each is a nucleated mass of cytoplasm from which several fine, branched processes project. The processes of different nerve cells join at their ends so as to form a nervous network lying on the mesoglcea, and connected with the muscle-tails of the musculo-epithelial cells.

Sense Cells

Here and there between the musculo-epithelial cells are narrow sense cells which each have a small rod-like projection at the outer end. From the base of each sense cell a cytoplasmic thread runs to join the network of the nerve cells. The Reproductive Organs

An examination of a cross section of Hydra through a reproductive organ will show it to be a mass of cells enclosed by a tightly stretched layer of the ectoderm cells. In the case of a testis the cells inside are extremely small but possessed of prominent nuclei which give the section a dark colour there. In the ovary the cells are much larger, being yolky, and in a ripe ovary one has developed at the expense of the others.

The Endoderm

The endoderm cells are taller than those of the ectoderm, and are of two main kinds, named respectively the secretory and nutritive cells. The Secretory Cells

These are most numerous on the hypostome and upper part of the column, and are not found on the foot or tentacles. They form digestive enzymes which are passed into the enteron when this contains food. Each cell narrows towards the base and consists of granular cytoplasm containing a large nucleus and enclosing a large vacuole.

The Nutritive Cells

The cells are columnar and at their bases are muscle-tails similar to those of the musculo-epithclial cells of the ectoderm. The muscle-tails of the nutritive cells, however, run around the column, and their contraction causes the body to become longer and narrower. The ends of the nutritive cells which abut on the enteron bear a pair of flagella, or are produced into pseudopodia.

Nerve Cells

These are similar to those in the ectoderm, forming a network on the inner side of the mesogcela connected with the outer network and with the muscle-tails of the nutritive cells.

Sense Cells

These are relatively fewer than those in the ectoderm and are connected similarly with the nerve net.

In the cytoplasm of the nutritive cells there are large vacuoles, and there are also granules called plastids, which multiply by binary fission. In the brown Hydra the plastids are yellow, and in the Common Hydra they are colourless. In the nutritive cells of the green Hydra instead of plastids there are small green plants called Zoochlorellas. Each plant is unicellular, consisting of a small nucleated mass of protoplasm surrounded by a cell wall and containing the green pigment chlorophyll. By living inside the Hydra, the Zoochlorella? obtain protection and the raw materials necessary for building up their protoplasm. The Hydra profits by obtaining sugar and oxygen from the plants, and thus it is beneficial for both organisms to live together. Such a partnership of two organisms living together for their mutual benefit is termed Symbiosis.


The tentacles of Hydra are provided with stinging cells which are used as weapons of offence and defence. Hydra feeds on small animals in the water, especially those minute crustaceans known as water fleas. If a swimming water flea bumps into the tentacles of a Hydra it is immediately held and paralysed by the nematocysts, and then passed by the tentacles to the mouth, which is opened widely to receive it. After digestion by means of enzymes in the enteron, the food is absorbed by the nutritive cells.


Hydra can swim by undulating movements of its tentacles, and even when it is fixed to some water plant it is continually bending and swaying its body in the water. It can move slowly along the surface to which it is attached by contractions of the foot, and is able to move more quickly by ’looping, ’ somewhat after the fashion of a looper caterpillar. The body is bent over until the tentacles touch the surface to which it is fixed, so that the animal is bent in the shape of a loop. Next the foot relaxes its hold and moves up to the tentacles where a fresh grip is taken. Then the tentacles are released, and the animal has in this way altered its position. By the repetition of this process Hydra can move slowly along a piece of water weed. Sometimes when the body is in the arched position, the foot, instead of taking a fresh grip close to the tentacles, turns a somersault so as to gain a new hold on the far side of the tentacles. The somersaulting method of progression takes Hydra along the stem more quickly than the looping method.


Hydra is sensitive to light and moves from dark places into the light, as may be shown if specimens are kept in an aquarium which is lighted on one side. The animals will collect on the.bright side of the aquarium, and if this is turned round they will move across to the well-lit side. It contracts when touched.


Hydra is given its name because of its power of regrowing or regenerating lost parts of its body. If any tentacles are lost they are replaced, and if the animal is cut into two or even into many small pieces, a new organism eventually grows from each piece.

The Budding of Hydra

Under natural conditions when the food supply is good, before ponds have begun to be stagnant, Hydra will reproduce by budding. A solid bulge appears half-way along the body. This elongates, the hollow forming within being continuous with the enteron of the parent. Tiny buds appear at its end which grow out into tentacles. The base of the bud now begins to narrow down until it closes the connection between the enteron of the parent and its bud. At this time the mouth of the bud breaks through, so that it is no longer dependent upon the parent for food. A sudden contraction causes the bud to break away, and it swims off to settle down elsewhere.

The Sexual Reproduction of Hydra

The different species of Hydra exhibit sexual reproduction at different seasons—H. fusca in the autumn, H. viridis and H. vulgaris in spring and summer. In all cases the process is similar. Each organism can develop both male and female parts and is therefore hermaphrodite, but as the two kinds of organs are not usually ripe at the same time, self-fertilization is rare. The testes usually develop first as two or three bulges just below the ring of tentacles. These are due to the rapid multiplication of interstitial cells pushing out the ectoderm. When ripe, the cells develop a flagellum each, forming sperm cells which burst the ectodermal covering at a particular spot and swim to ripe ova. As a rule only one ovary develops on a Hydra, at the lowest third of the body where a bulge appears, again due to the multiplication of the interstitial cells, one of which becomes amoeboid and proceeds to feed upon the others, growing large and laying down yolky granules of reserve material. When only this ovum remains in the ovary, it withdraws its pseudopodia and becomes spherical, the ectodermal wall gives way, and the ripe ovum hangs free in the water, suspended from the parent by a strand of gelatinous material which also surrounds it. If fertilized by a sperm at this stage, it secretes a thick wall of chitin and frees itself from the parent and sinks to the bottom of the pond. Later, the covering splits and the young animal emerges. It is a hollow mass of cells, one end of which becomes fastened down whilst the other develops a mouth and tentacles.

Practical Work on Hydra

Examine Hydra in water in a watch-glass with a hand lens.

Mount Hydra in a drop of water on a slide, and in order to prevent crushing the animal support the coverslip by two small slips of glass. Wait until the animal is expanded and then examine with the low power of the microscope.

Mount Hydra in a drop of water under a coverslip in the usual way and examine one of the tentacles under the high power. Draw 1 per cent, acetic acid under the coverslip and examine for discharged nematocysts. Press the coverslip gently so as to squash the animal and try to make out the different kinds of cells.

Examine prepared sections of Hydra— in longitudinal section, showing ovary, in transverse section, showing ripe ovum. showing testes,

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