body at each step. Sometimes the body goes end over end in slow somersaults.
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.
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Fig. 40.—Hydroid Colony, with nutritive (P) reproductive (M) and defensive (S) hydranths.
The Branch Polyps (sometimes called Cœlenterata).—The hydra is the only 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.
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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.
Hydroids.—Figure 40 shows a hydroid, or group of hydra-like growths, one of which
