134 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



gus (oe), the globular stomach («), and the ventral intes- 

 tine (^), are very similar to those of the pluteus. 



The ciliated ridges (i and 2) before and behind the 

 mouth, are prolonged into a number of paired lateral 

 arms, but these are shorter and more numerous than those 

 of the pluteus, and they have no supporting skeleton. 



In the pluteus the ciliated ridge which passes in front 

 of the mouth, fringes the pre-oral arms, and then, run- 

 ning back onto the lateral arms, fringes the post-oral arms, 

 and then passes across the ventral surface behind the 

 mouth, so that its course forms a single closed circlet. 



In the bipinnaria the pre-oral ciliated ridge (i), after 

 fringing the pre-oral arms (a'), runs forward on each side 

 to form a lobe (Fig. 78, 3) on the ventral surface of the 

 large oral lobe (a). It thus forms a small closed circlet on 

 the ventral surface in front of the mouth, and encloses a 

 surface which is entirely ventral, and which is known as 

 the pre-oral plastron. 



The ciliated ridge (2), which passes between the mouth 

 and the anus, becomes bent into a pair of arms (c), which 

 answer to the post-oral arms of the pluteus. It then runs 

 backwards on each side to form a pair of lateral arms (7i), 

 and then runs forward along the edges of the dorsal sur- 

 face (Fig. 79), folding out to form two pairs of lateral 

 dorsal arms (q and q'). The two sides finally meet at the 

 tip of the dorsal surface of the anterior lobe («), where 

 they form an unpaired lobe (Fig. 79, 4). This circlet 

 surrounds an area partly ventral and partly dorsal, and 

 known as the anal plastron. There are thus two closed 

 circlets of cilia in the starfish larva, instead of one as in 

 the sea-urchin, and one of these is in front of the mouth 

 and on the ventral surface, while the other runs between 

 the mouth and the anus, and fringes the dorsal surface. 



