418 



EMBRYOLOGY 



anterior, so-called preoral, and a posterior, anal area can be 

 distinguished as elevated parts from the depressed portions 

 (Fig. 198 G and D). At the anterior and posterior ends the 

 two areas bend around toward the dorsal surface. Fig. 

 199 ^ shows a larya at about this stage as seen from the 

 side. The further development of the shape of the larva 



is finally attained by 

 the extension of the 

 depressed, or hollowed- 

 out, region more to the 

 periphery, and by the 

 production of lobular 

 processes at the mar- 

 gins of the body, owing 

 to the outgrowth of cer- 

 tain parts (Fig. 198 D). 

 Calcareous deposits, 

 having the shape of 

 delicate miniature 

 wheels, may make their 

 appearance in these 

 ear -like appendages 

 (Fig. 205, p. 426). Along the periphery of the lobes runs an 

 uninterrupted ciliated band, which borders the two ventral 

 areas as well as the dorsal surface. 



In each of the two depressed lateral surfaces of the Auricularia larva 

 lies a structure resembling a ciliated band ; but these structures bear no 

 relation to the ciliated band itself. Each of these two bands exhibits the 

 form of a blunt angle opening toward the ventral surface. The cords 

 consist of ciliated cells and fine longitudinal fibres lying under them. 

 Strands of fibres pass from them to the ciliated band. Accordingly 

 Metschnikoff (No. 37) and Semon (No. 55) interpret the two cords as the 

 central nervous system of the larva. They also occur under similar cir- 

 cumstances—to anticipate— in the Pluteus larvae of the Ophiuroidea. 

 On the other hand, corresponding structures do not occur in the larvae of 

 the Echinoidea and Asteroidea. According to Semon, however, fine 

 fibres, similar to those in the nerve cords of the Auricularia larvae, occur 

 in the ciliated band of these larvae, so that the nerve apparatus would be 

 connected with the ciliated bands in the same way as in the larvae of the 

 Annelida (comp. p. 266). 



[A very large Holothurian larva (Auricularia nudibranchiata), which 



Fis. 199.— -4 and B, larva of a Holothurioid 

 and an Asteroid respectively seen from the side 

 (from Balfour's Comparative Emhryology] , a, 

 anus ; l.c, ciliated band ; m, mouth ; pr.c, adoral 

 band of cilia of the Bipinnaria ; st, stomach. 



