THE HIGHER ORGANISMS 



127 



The elementary composits com- 

 posed of fairly homogeneous ele- 

 ments possess no special motor or- 

 gans. Their cells may be amoe- 

 boid as in microgromia, or they 

 may be ciliated as in volvox, the 

 cilia being so disposed as to serve 

 the best interests of the colony. 

 In volvox they are placed extern- 

 ally to permit movement; in the 

 porifera, which are immobile, the 

 ciliated cells are internally disposed 

 so that their lashing produces cur- 

 rents of water which constantly 

 flow through the radiating canals 

 carrying in the minute particles 

 upon which the amoeboid cells of -* v~ 

 the entoderm seize. 



True prehensile organs, composed 

 of many cells, first make their ap- 

 pearance among the ccelenterates, 

 and are most simple in hydra, 

 where they form a circle of from 

 six to ten long slender arms about 

 the oral aperture. Each of these 

 arms or tentacles has a structure 

 corresponding with the body of the 

 animal itself, that is, it consists 



FIG. 44. Enlarged view of the anterior and 

 posterior parts of the body of an earthworm as 

 seen from the ventral aspect, an, Anus; c, clitel- 

 lum; g.p., glandular prominences on the twenty- 

 sixth somite; in, mouth; o.d, external openings 

 of the oviducts; p.s, prostomium; s, setae; s.r, 

 openings of the seminal receptacles; s.d, external 

 openings of the sperm ducts. The form of the 

 body varies greatly in life according to the state 

 of expansion. The specimen here shown is from 

 an alcoholic preparation (slightly enlarged). (Sedg- 

 wick and Wilson.) 



