In what ways does the hydra show “=division of labor=”? 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 _ameboid motion_, that is, they extend pseudopods. They also resemble amebas 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. 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. [Illustration: FIG. 40.--HYDROID COLONY, with nutritive (_P_) reproductive (_M_) and defensive (_S_) hydranths.]