204 



COLLEGE ZOOLOGY 



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

 (Fig. 142, i) 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. 



com p. el v. 



amp 



g.rach 



gen 



bv. 



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

 pullae 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; reel., rectum; 

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

 the Cambridge Nat'ural 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." 



