ZOOLOGY. 



function in this group is not differentiated from the digestive. 

 In the colonial forms the gastro-vascular cavity of the various 

 polyps in the colony may be directly continuous (Fig. 85). 

 In the medusa, the corals, and forms like anemone, the cavity 

 ic? much more complicated than in the tubular hydroids, on 

 account of the mesenteries. The entoderm seems to take up 

 food from the gastro-vascular cavity, in part at least, by means 

 of the amreboid action of some of the entodermic cells. 

 Pseudopodia are formed, and particles are directly taken into 

 the body of the cell. Special gland cells also occur in the 

 entoderm, by the secretions of which the food undergoes 

 changes preparatory to absorption. There is no anal opening. 

 Undigested remnants are eliminated at the mouth. Respira- 

 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 like- 

 wise a general body function. 



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

 tractile fibres. Many of these are modified ectodermal or 

 entodermal cells rather than true mesoderm (Fig. 79, 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 con- 

 tractions 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. 



221. 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. This 



