Chap. XIV.] 



LARV& OF ECHINOZOA. 



537 



to the exterior (Fig. 222; wp). The vesicle then breaks 



up into three parts, the most anterior of which gives 



rise to the water-vascular ring and its appended canals, 



while the two more posterior (Fig. 222 ; rp, Ip) give rise 



to the general body 



cavity, the lining of 



which is formed by 



their walls. In Holo- 



thurians generally, 



though not always, the 



connection between the 



vascular system and the 



body wall becomes 



broken, and the madre- 



poric canal hangs freely 



in the body cavity. 



Among other Echi- 

 nozoa the amount of 

 difference between the 

 larval and the adult 

 stage is much greater 

 than it is in Holothu- 

 rians ; the larvae are 

 more elaborately de- 

 veloped, and present 

 distinct evidences of 

 secondary adaptations 

 to their free mode of 

 life. The sides of the 

 body are not unfre- 



quently produced into free arm-like processes, the 

 interior of which may (Pluteiis larvae, Fig. 223), 

 or may not (Brachiolaria), be supported internally 

 by delicate calcareous rods. Part only of the body of 

 such larvae passes directly into the substance of the 

 adult ; the rest is either -absorbed by the growing 

 echinozoon, or shrivels up and disappears. The 



Fig. 222. Diaenrammatic View of the 

 J arva of a Holothuriaa (from the 

 side). 



m, Mouth ; g, gullet ; s, stomach ; a, anus ; c, 

 longitudinal ciliated band ; w, rudiment 

 f water-vascular ring; wp, water-pore; 



rp, Ip, right and left peritoneal cavities, 

 from which the body cavity is developed. 

 (From P. H. Carpenter, after Beleoka ) 



