Fig. 39.—Longitudinal section of hydra (microscopic and diagrammatic).
Compare the sponge and the hydra in the following respects:—many celled, or one-celled; obtaining food; breathing; tubes and cavities; openings; reproduction; locomotion. Which ranks higher among the metazoa? The metazoa, or many-celled animals, include all animals except which branch?
Figure 39 is a microscopic view of a vertical section of a hydra to show the structure of the body wall. There is an outer layer called the ectoderm, and an inner layer called the endoderm. There is also a thin supporting layer (black in the figure) called the mesoglea. The mesoglea is the thinnest layer. Are the cells larger in the endoderm or the ectoderm? Do both layers of cells assist in forming the reproductive bud? The ectoderm cells end on the inside in contractile tails which form a thin line and have the effect of muscle fibres. They serve the hydra for its remarkable changes of shape. When the hydra is cut in pieces, each piece makes a complete hydra, provided it contains both endoderm and ectoderm.
In what ways does the hydra show “division of labour”? Answer this by explaining the classes of cells specialized to serve a different purpose. Which cells of the hydra are least specialized? In what particulars is the plan of the hydra different from that of a simple sponge? An ingenious naturalist living more than a century ago, asserted that it made no difference to the hydra whether the ectoderm or the endoderm layer were outside or inside,—that it could digest equally well with either layer. He allowed a hydra to swallow a worm attached to a thread, and then by gently pulling in the thread, turned the hydra inside out. More recently a Japanese naturalist showed that the hydra could easily be turned inside out, but he also found that when left to itself it soon reversed matters and returned to its natural condition, that the cells are really specialized and each layer can do its own work and no other.
Habits.—The hydra’s whole body is a hollow bag, the cavity extending even into the tentacles. The tentacles may increase in number as the hydra grows but seldom exceed eight. The hydra has more active motion than locomotion. It seldom moves from its place, but its tentacles are constantly bending, straightening, contracting, and expanding. The body is also usually in motion, bending from one side to another. When the tentacles approach the mouth with captured prey, the mouth (invisible without a hand lens) opens widely, showing five lobes or lips, and the booty is soon tucked within. A hydra can swallow an animal larger in diameter than itself.
The endoderm cells have amœboid motion, that is, they extend pseudopods. They also resemble amœbas in the power of intra-cellular digestion; that is, they absorb the harder particles of food and digest them afterwards, rejecting the indigestible portions. Some of these cells have flagella (see Fig. [39]) which keep the fluid of the cavity in constant motion.
Sometimes the hydra moves after the manner of a small caterpillar called a “measuring worm,” that is, it takes hold first by the foot, then by the tentacles, looping its body at each step. Sometimes the body goes end over end in slow somersaults.
Fig. 40.—Hydroid Colony, with nutritive (P) reproductive (M) and defensive (S) hydranths.