422 EMBRYOLOGY 



one of these occasionally divides, a certain resemblance 

 to the Braehiolarian arm is pi'oduced. For in this way 

 there arise two lobes, which lie symmetrically in front 

 of the mouth, and a third unpaired lobe, which is farther 

 removed from the mouth-opening. But the arms of the 

 Brachiolaria which lie in front of the mouth have a similar 

 position, and therefore Ludwig homologizes the larval 

 organs of Asterina with the latter. The organ is muscular, 

 and serves the larva for attachment. Similar, but multifid, 

 larval appendages have also been described by Saks, Joh. 

 MtJLLER, Agassiz, THOMPSON, and others, for Echinaster and 

 Asleracanthion (AI idler i) . 



Nothing more detailed has been learned of a vermiform 

 Echinoderm larva described by Joh. MOlleb, which was 

 divided into five segments by transverse constrictions, and 

 to the nndei'-surface of which a five-lobed star was attached. 

 According to Joh. Muller's account, it develops into a star- 

 fish. 



Ophiuroidea. — The Pluteus larvoeof the Ophiuroidea e.x- 

 hibit an es.sentially different shape from that of the larvae 

 thus far considered. But they also arise from the same 

 fundamental form. As in the cases previously considered, 

 there is a continuous ciliated band, which boi'ders the deep 

 depressions of the body (Fig. 202 A). The subsequent 

 characteristic shape of the larva depends, in the first place, 

 upon the fact that the anal ai-ea increases considerably in 

 extent, while the preoral ai'ea, on the contrary, almost 

 entirely disappears (Fig. 202 i>). Apart from this, the 

 shape of the larva is determined by the long processes into 

 which its peripheral portions grow out (Fig. 202 G and D). 

 These are bordered by the ciliated band, which is still, and 

 always remains, continuous. As the form of the Plutens is 

 reached, the anal area becomes pointed (Fig. 202 D). The 

 two ventral, posterior arms are especially well developed. 

 They are also significant for the reason that they are always 

 present, whereas the other arms may be more or less sup- 

 pressed. 



The Pluteus larvae, unlike the Aaricularia and Bipinnaria, 

 possess a calcareous skeleton. As early as the gastrula 



