508 



COMPARATIVE ANATOMY 



CHAP. 



that portion ; it forms a moat round the preoval area, which becomes raised up like a 

 shield. 



An adoral ciliated ring, closely encircling the mouth and extending some way 

 into the buccal cavity, is also present. 



The body is produced into longer or shorter processes or arms, in the regions of the 

 preoral and circumoral ciliated rings. An anterior unpaired frontal process, belong- 

 ing to the ciliated ring, is distinguished by its constant occurrence and its greater 

 length. 



In some .species, the ciliated band disappears on this frontal process, which, on 



Fig. 399. — Older Auricularia, seen diagonally from the lower and left side (after Semon). 1, 

 Circumoral ciliated ring ; -2, hydropore ; 3, liydrocfel ; 4, adoral ciliated ring ; 5, median or stomacli 

 intestine; 6, nerve band ; 7, hind-gut ; S, left euteroccel ; 9, calcareous wheel ; 10, fore-gut, ceso- 

 phagus ; 11, right euterocod. 



the other hand, divides into three branches, apparently covered with protuberances 

 at tlieir tips. Such larvae are known as Brachiolarise. 



There are, further, Asteroids whose larvte do not at all resemble the Bipimiariau 

 and Bracliiolarian larvffi, or else show only a superficial resemblance to them ; (/. 

 below the account of the larva of Asterina gibbosa (p. 525). 



3. Ophiuroidea. — The Ophiurid larva is known as the Pluteus, and can be 

 just as easily deduced from the hypothetical larval form of the Echinoderms, sketched 

 above, as the AiirictiJiiria and the Bipinnaria. The gastrula stage is followed by 

 the bilateral stage with depressed ventral surface, in the centre of which lies the 

 larval mouth. A circumoral ciliated band is retained, running along the edge of this 

 ventral depi ession. This band always remains single. "While the preoral area (the 

 larva being viewed from the ventral side) remains very small or is even indistinguish- 

 able, the anal area appeals very large. The body is produced into processes or arms, 

 which may become very long, and are supported by calcareous rods. These pro- 



