204 



COLLEGE ZOOLOGY 



testine part way, opening into it at either end. The anus 

 (Fig. 142, j) of the sea-urchin is near the center of the aboral 

 surface. 



Respiration. — A large part of the respiration takes place in 

 most echinoids through ten branched pouches situated on the area 



comp.ret. 

 k-fe^^ romp.elv. 



bv. 





prot. 



oe 



tooth sac 



Fig. 143. — Internal anatomy of a sea-urchin, Echinus esculentus. amp., am- 

 pulla of tube-feet; aur., auricle; b. v., " dorsal blood-vessel "; comp., " com- 

 passes" of Aristotle's lantern; comp. eh., elevator muscles; comp. ret., retractor 

 muscles; eph., epiphyses of jaws; gon., gonad; g.rach, genital rachis; int, in- 

 testine; oe, oesophagus; prot., protractor of Aristotle's lantern; rect., rectum; 

 ret., retractor muscle; siph., siphon; st., stomach; stone c, stone-canal. (From 

 the Cambridge Natural History.) 



surrounding the mouth, one pair in each angle between the 

 ambulacral plates. The tube-feet also are respiratory in 

 function. 



Locomotion. — Both tube-feet and spines are used in loco- 

 motion. " The spines are pressed against the substratum and 

 keep the animal from rolling over under the pull of the tube- 

 feet and also help to push it on." 



