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at all depths and in water of temperatures of varying degrees. The same species of sponge in different localities may assume very different shapes, the immediate surroundings acting upon the animal so as to change its form. They appear to be protected from fish and other animals because of their color and form, their skeleton, and an unpleasant odor.
Miner, A Guide to the Sponge Alcove. Guide Leaflet, No. 23. American Museum
of Natural History, New York.
Parker and Haswell, Text-book of Zoology. The Macmillan Company.
The sponge, as we have seen, is an animal of simple form and of a low order in the scale of life. Another simple animal is the hydra - a type of the group Colenterata. The group Catenterata includes animals which have a common food tube and body cavity.
The common hydra lives in fresh water. It may be found living on dead leaves, submerged sticks, stones, or weeds in almost any fresh-water pond. In a small fresh-water aquarium, they almost always
attach themselves to
the side of the jar, where they can be watched and their movements observed.1
Notice that when contracted they resemble a little whitish ball of jellylike substance. When undisturbed they elongate into a hollow cylinder attached at one end. A small oval opening is found at the free end, surrounded with a number of little waving arms called tentacles. Count them to see
Budding hydra, as seen under the low power of a compound microscope; B, attached end; B1, B2, buds; M, mouth; T, tentacles.
how many there are. If by chance a
small water flea or other crustacean on which the hydra feeds
1 See Hunter and Valentine, Manual, page 151.
happens near enough to touch one of the tentacles, it is seen to stop suddenly as if paralyzed and is then grasped by the tentacle and carried toward the opening. The hydra is simply a hollow bag, the digestion of the prey taking place within it. The waste products are passed out through the opening between the tentacles where food is taken in.
The hydra may change its position from time to time. The animal bends over, the base is unfastened, and the animal "walks " on its tentacles to a new point of attachment.
STRUCTURE OF BODY WALL. -The body wall of the hydra seen in a cross section is found to be made up of two layers of cells. As in the sponge, the inner layer serves for the purpose of taking in and digesting the food. This layer is the endoderm. The outer layer of cells (called ectoderm) furnishes the animal with weapons of offense and defense. This outer layer is also provided with cells which are sensitive (sense cells). Between the inner and outer layers of cells is a structureless substance called the mesoglea, in which are found musclelike fibers, extensions of the protoplasm of the inner outer layer. Scattered among these musclelike fibers are some of the cells of the outer wall, irregular in shape, which have migrated in from the outer layer. These cells are the nerve cells. They furnish the animal with pathways of sensation and provide a means of coördinated movement.
Organs of Offense and Defense. If the cells of the outer part of the tentacle are examined under the microscope, we find how the animal is able to paralyze its prey. Here are found many cells, the bodies of which resemble little bags. One end of the S cell projects from the outer surface of the tentacle. These cells are the cnidoblasts, or stinging cells. Each cell has in its body a small sac filled with an acid. Attached to this bag and rolled up in it is a long hollow dart, which can be expelled when the cell is " set off." This is done by an animal or substance brushing against a triggerlike projec
Stinging cells (cnidoblasts); cnc., cnidocil; d., dart; n., nucleus; s., sac. (Drawing, greatly enlarged, after Parker and Haswell.)
tion from the cnidoblast. The "explosion " of the cell results in the ejection of the dart and injection of the poisonous acid into the victim. The animal hit by a number of these darts is usually so paralyzed that further resistance is impossible.
Food Taking. The tentacles then reach out like arms, grasp the food, and bend over with it toward the mouth. Digestion takes place by means of a fluid given out from some of the cells near the mouth. After the food has been partially digested, many of the cells lining the cavity. put out pseudopodia, which grasp and ingest the food particles. The tentacles are hollow, and the body cavity extends into them. From this cavity food may be taken up by cells in all parts of the body near where it is to be used. The middle and outer layers of the animal do not digest the food, but receive some of it already
digested from the inner Longitudinal section of a hydra; b, bud; ba, attached layer. This food passes end; m, mouth; ov, ovary; sp, spermary holding from cell to cell, as in plants,
by osmosis. The oxygen necessary to oxidize the food is passed through the body wall, seemingly at any point, for there are no organs for respiration (breathing).
Division of Labor. We have here then a step toward a more complex animal, for certain parts of the body here have certain work to perform. The outside for sensation, offense, and defense, the middle layer for movement and coördination of parts, and the interior of the bag for taking in food, digesting it, and distributing it to other parts of the body.
The hydra reproduces itself either by budding
or by the production of new animals by means of eggs and sperms, sexually. The bud appears on the body as a little knob, sometimes more than one coming out on the same hydra. At first the bud is part of the parent animal, the body cavity extending into it. After a short time (usually a few days) the young hydra separates from the old one and begins life anew in another place. This is asexual reproduction.
The hydra also reproduces by eggs and sperms. These sperms are collected in little groups which usually appear near the free end of the animal, the egg cells developing near the base of the same hydra. Both eggs and sperms grow from the middle layer of the animal. The sperms when ripe are set free in the water, one of them unites with an egg, which is usually still attached to the body of the hydra, and development begins which results in the growth of a new hydra in a new locality.
Medusa. The cœlenterate animals are a very large group and contain many other animals than the hydra. All of this group of
animals are found in the
Medusa (Gonionemus murbachii), showing tentacles,
water, by far the greatest number of forms living in the ocean. Among the most interesting of all the cœlenterates inhabiting the salt water are the jellyfishes or medusæ.
For the study of the Medusae1 use Gonionemus preserved in formol. Why should you call it a " jellyfish"? Notice the shape of the animal, somewhat like an umbrella with a short handle. What do you
find hanging from the edge of the umbrella? The tentacles
of the medusa are provided with stinging cells. Find and describe the mouth at the end of the handle of the umbrella (manubrium). (N.B. A little carmine or methylene blue dissolved in water may be forced into the mouth of the specimen with a small medicine dropper. This will show clearly the digestive cavity and its canals). Notice the course taken by the canals forming the digestive tract. Hanging just under the radial canals of the digestive tract are found the reproductive bodies, eggs or sperms.
1 See Hunter and Valentine, Manual, page 156.