The length of the extended hydra may reach one half inch. When touched, both tentacles and body contract until it looks to the unaided eye like a round speck of jelly. This shows sensibility, and a few small star-shaped cells are believed to be nerve cells, but the hydra has not a nervous system. Hydras show their liking for light by moving to the side of the vessel or aquarium whence the light comes.
Fig. 41.—“Portuguese Man-o’-war” (compare with Fig. [40]). A floating hydroid colony with long, stinging (and sensory) streamers. Troublesome to bathers in Gulf of Mexico. Notice balloon-like float.
The Branch Polyps (sometimes called Cœlenterata).—The hydra is the chief fresh-water representative of this great branch of the animal kingdom. This branch is characterized by its members having only one opening to the body. The polyps also include the salt water animals called hydroids, jellyfishes, and coral polyps.
Hydroids.—Figure 40 shows a hydroid, or group of hydra-like growths, one of which eats and digests for the group, another defends by nettling cells, another produces eggs. Each hydra-like part of a hydroid is called a hydranth. Sometimes the buds on the hydra remain attached so long that a bud forms upon the first bud. Thus three generations are represented in one organism. Such growths show us that it is not always easy to tell what constitutes an individual animal.
Fig. 42.—The formation of many free-swimming jellyfishes from one fixed hydra-like form. The saucer-like parts (h) turn over after they separate and become like Fig. [43] or 44. Letters show sequence of diagrams.
Hydroids may be conceived to have been developed by the failure of budding hydras to separate from the parent, and by the gradual formation of the habit of living together and assisting one another. When each hydranth of the hydroid devoted itself to a special function of digestion, defence, or reproduction, this group lived longer and prospered; more eggs were formed, and the habits of the group were transmitted to a more numerous progeny than were the habits of a group where members worked more independently of one another.
As the sponge is a simple example of the devotion of special cells to special purposes, the hydroid is a primitive and simple example of the occurrence of organs, that is of special parts of the body set aside for a special work.
