l82 ZOOLOGY 



tion the exchange of carbon dioxide for oxygen takes place 

 by means of the individual cells of the body layers, though it 

 is probable that it takes place more satisfactorily in the thin- 

 walled, more actively moving tentacles. Excretion is likewise 

 a general body function. 



225. Motion. All the Ccelenterata are supplied with con- 

 tractile fibres. Many of these are modified ectodermal or ento- 

 dermal cells rather than true mesoderm (Fig. 81, B). The 

 fibres run both longitudinally and transversely. In the more 

 active types cross-striate fibres may occur. The attached 

 (polyp) forms have well-developed longitudinal fibres in the 

 body wall and the mesenteries, which enable the soft parts of 

 the animal to be drawn close to the supporting object. In 

 the medusoid types locomotion is effected by rhythmic contrac- 

 tions of the bell as a whole. By this means the water is expelled 

 from the cavity of the bell, and the reaction forces the animal 

 forward. 



226. Support. The attached colonial forms (corals, sea- 

 fans, etc.) usually possess a skeleton of calcareous or horny 

 matter commonly secreted by the ectoderm. Each polyp con- 

 tributes a portion to the common skeleton the corallum. The 

 corallum differs greatly in form in the different species. The 

 particular form depends on the manner and rate of budding or 

 non-sexual reproduction of the polyps, and the activity shown 

 by the individual in secreting. In some cases single polyps 

 produce a skeleton (cup-corals). The coral reefs of tropical 

 seas are illustrations of the power of corals to form and excrete 

 carbonate of lime. Much of the lime-stone of the earth's crust 

 shows that corals assisted in its formation. 



227. Sensation and Behavior. The nerve cells may be 

 scattered diffusely over the surface of the body with a mesh- 

 work of fibrils to connect them with the muscular and nettle 

 cells and with each other, as in Hydra. In some other polyp- 

 forms there is more differentiation of cells and fibres, but the 

 elements are still scattered. In the more active types there is 

 a collection of the cells either as a connected ring, or in groups, 



