HYDRA

LABORATORY EXERCISE

Technical Note.—Hydra lives in fresh water, attached to stones, sticks, or decayed leaves. It can be found in most open fresh-water ponds not too stagnant, often attached to Chara. There are two species occurring commonly, H. viridis, the green Hydra, and H. fuscus, the brown or flesh-colored Hydra. Both are very small forms and have to be looked for carefully. Specimens should be brought to the laboratory, put into a large dish of water and left in the light. Hydra is best studied alive. Place a living specimen attached to a bit of weed in a watch-crystal filled with water or on a slide with plenty of water and examine with the low power of the microscope.

Note the cylindrical body (fig. [7], A, B) with its flat basal attachment and radial tentacles (varying in number) which crown the upper end and surround the centrally located mouth. Note the movements of Hydra, its powers of contraction, and method of taking in food.

Technical Note.—To feed Hydra, place very small "water-fleas" (Daphnia sp.) in the water with it.

Observe the method by which "water-fleas" are taken into the mouth. Food is caught on stinging cells (to be studied later) and conveyed to the mouth by the tentacles. Note that the cylindrical body encloses a cavity, the digestive cavity. How is this connected with the exterior? If Hydra captures prey too large or is no longer hungry, the prey is released.

Fig. 7.—A, Hydra fusca, with expanded body and a budding individual; B, H. fusca, contracted; C, H. fusca, part of outer surface of a tentacle, greatly magnified. (A and B drawn from live specimens, C, from a preparation) D, Grantia sp. (a sponge), three individuals; E, Grantia sp., longitudinal section; F, Grantia sp., spicules. (D, E, and F drawn from preserved specimens.)

Technical Note.—Place small slips of paper on the slide near the Hydra, put cover-glass over the whole, and examine with the low power of the microscope.

Note that the whole animal is made up of cells closely joined. Are the cells in the tentacles all alike? Note nodule-like projections above some of the cells; these are stinging cells, or cnidoblasts. In some cases a small hair-like process, the trigger hair or cnidocil, may be seen projecting above the surface of the cell. Note in some of the tentacles dark-colored particles. These are food-particles which have been taken through the mouth into the digestive cavity and have passed thence into the tentacles. The central digestive cavity communicates freely with the cavities in the tentacles, for the tentacles are merely evaginations of the body-wall.

Make drawings of the Hydra expanded and of the same individual contracted.

Technical Note.—From the preparation which you have under the microscope pull out the slips of paper, thus letting the cover-glass drop down on the specimen. With a small pipette put a drop of anilin-acetic stain (see p. [451]) on the slide at one side of the cover-glass and with a piece of filter-paper draw the water through from the other side of the cover-glass. When the stain is diffused press down the cover-glass gently and examine the tentacles first under a low power of the microscope, then under a high one.

Note the distortion that the animal has undergone through the action of the reagent. Observe the cnidoblasts of the tentacles and note that many of them have thrown out long whip-like processes (fig. [7], C). On what parts of the body do the cnidoblasts occur? Carefully examine one of the cnidoblasts which has been discharged and note a clear transparent bag-like structure within, the nematocyst, to which is attached the long whip-like process. In another cnidoblast cell which has not been discharged note that the whip-like process is coiled about inside of the bag-like structure. The whole apparatus is like the inturned finger of a glove which can be blown out by pressure from the inside. The mechanism is simple. The cnidocil or trigger-hair is touched by some animal, an impulse is conveyed to the delicate fibres interspersed among the cells (nerve-cells) which stimulate the cnidoblast cell, whereupon there is a contraction of the contents and, the cnidoblast being compressed, the inverted whip-like process turns wrong side out and impales the animal on its points or barbs.

Technical Note.—The teacher should be provided with microscopical sections, both transverse and longitudinal, of the Hydra stained in some good general stain (hæmatoxylin or borax carmine). If the teacher has no means of making such preparations, they may be procured from dispensers of microscopical supplies.

From the cross-section of the Hydra make out the general structure of the body. Note that it is a hollow cylinder consisting of two well-defined layers of cells, an outside ectoderm layer and an inner endoderm layer. Between these two is yet another thin non-cellular layer called the mesoglœa.

Thus it will be seen that Hydra is made up of two layers of cells, the outer ectoderm or skin, which is specialized to perform the office of capturing prey as well as that of protection, and the inner endoderm, which surrounds the digestive cavity and performs the function of digestion. The endoderm lines the body-cavity, particles taken in as food being digested by certain digestive cells which thrust out amœboid processes and ingest particles of food. Other cells in the endoderm have long flagellate processes which vibrate back and forth in the digestive cavity, thereby creating currents in the water containing food-particles.

Note, in a cross-section, that there are small ovoid or cuboid cells at the bases of the large ectoderm cells. These are the interstitial cells. Some of the interstitial cells become modified and pushed up between the ectoderm cells to form cnidoblast cells. Many of the endoderm as well as ectoderm cells have muscle-processes which spread out from the base of the cell and which serve to contract and expand the body.

Technical Note.—In the specimens which have been collected perhaps two methods of reproduction will be observed. Place healthy Hydræ in a wide-mouthed jar in the sunlight with plenty of water and food. In a few days active budding will take place.

Observe the method of reproduction in Hydra. Commonly the parent produces small buds, which at first are only evaginations of the body-wall, but which later develop tentacles and a mouth of their own. Subsequently the bud becomes constricted at the base, separates from the parent, and the young Hydra begins a distinct existence.

Another mode of reproduction takes place which, in distinction from the asexual method just mentioned, is called sexual reproduction. This last is the method common to most of the higher organisms. You may note that in some Hydræ there is a swelling or bulging of the ectoderm of the body-wall in the region just below the tentacles. These are the sperm-glands. Within these are produced sperm-cells which break away in great clusters to fertilize the ova, or eggs. Note a larger bulging of the body-wall nearer the lower end of the body which, under high power, has a granular appearance. This is the egg-gland, in which develops a single ovum or egg. The ovum breaks from its covering and is fertilized by sperm-cells from another individual. In forms like Hydra, where both sexes are represented in a single individual, the organism is termed monœcious or hermaphroditic. In connection with reproduction Chapter [XIII] should be studied.

An instructive experiment can be performed by cutting a Hydra into two or more parts, when (usually) each of the various parts will develop into a complete Hydræ. This process may be called reproduction by fission, but it rarely occurs naturally.

[CHAPTER XI]