508 



COMPARATIVE ANATOMY 



that portion ; it foniisa moat round the preoial area, wliich 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 jirocesses 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 liy its constant occurrence and its greatei- 

 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 ; •_', hydropore ; 3, liydrocwl ; 4, adoral ciliated ring ; 5, meilian or stomach 

 intestine ; (i, nerve banil ; 7, hind-gut ; 8, left enterocwl ; 9, calcareous wheel ; 10, fore-gut, ceso- 

 phagus ; 11, right euterocoel. 



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

 at their tips. Such larvaj are known as Brachiolarise. 



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

 and Braclnolarian larv.^e, or else show only a superficial resemblance to them ; cf. 

 below the accotmt 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 pioduced into processes or arms, 

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



