508 



COMPARATIVE ANATOMY 



CHAP. 



that portion ; it forms a moat round the preoral 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 



20- 



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

 Circumoral ciliated ring ; 2, hydropore ; 3, hydrocoel ; 4, adoral ciliated ring ; 5, median or stomach 

 intestine ; 6, nerve band ; 7, hind-gut ; 8, left enterocrel ; 9, calcareous wheel ; 10, fore-gut, oeso- 

 phagus ; 11, right enterocoel. 



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

 at their tips. Such larvae are known as Brachiolariae. 



There are, further, Asteroids whose larvae do not at all resemble the Bipinnarian 

 and Brachiolarian larvae, or else show only a superficial resemblance to them ; cf. 

 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 Auricularia 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 depression. 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 appears very large. The body is produced into processes or arms, 

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



