ANATOMY AND PnrsWLOGY OF ECTUNOBERMS 243 



-3^ ah.r-ms 



individual several different organs aid in breathing. In 

 sea-urchins, as in starfishes, there are hollow outgrowths of the 

 skin which are so thin that oxygen can be absorbed by them 

 from the surround- 

 ing water. In star- 

 fishes such out- 

 growths are very 

 numerous and 

 small ; in echinoids 

 there are a few 

 large ones near the 

 mouth. Respira- 

 tion probably also 

 takes place through 

 the walls of the am- 

 bulacral feet and in 

 the extra intestine, 

 as just described. 

 The class of holo- 

 thurians have an 

 extensive, much- 

 branched sac which 

 lies in the cavity of 

 the body and is 

 filled with water 



Fig. 23.3. — A section through a typical sea-urchin, 

 showing the mouth (mo.) and the jaws with the 

 muscles (mus.) that move them. The food canal 

 (ali.) has been cut at the Ijeginning of the intestine 

 (int.) and the great loop (cf. Fig. 234) of the latter 

 removed for simplicity. The rectum (red.) and 

 anus (an.) lie at the upper pole. Other parts that 

 are referred to are : ah. r. re.s-, circular vessel around 

 anus; amp, bladders at base of tube-feet (/./.) ; 

 aur, calcareous loops for attachment of muscles ; 

 cal, the body cavity ; int. ve.s, intestinal blood 

 vessel; mcut., madreporite and its canal (mad. 

 can.) ; nen>. r, nerve-ring about mouth; oc, eye- 

 Ijearing plate ; or. r. v, circular vessel around 

 mQuth ; plex, gland on madreporic canal ; pol. ves, 

 reservoir to water system ; rad. amh, radial am- 

 bulacral vessel ; rad. iie, radial nerve ; siph, a tube 

 on the intestine (cf. Fig. 234). From Parker and 



that is periodically Hasweli. 



renewed. This sac is called a " water-lung " or " respiratory 



tree." 



Organs of circulation are ill defined. Great spaces or 

 cavities occur in the body which frequently surround the 

 other organs. These spaces contain sea-water plus a certain 



