554 



AURICULARIA. 



A similar ciliated ridsre is 



o 



ridge a little in front of the anus, 

 present on the ventral surface 

 of the prae-oral lobe immedi- 

 ately in front of the mouth. 

 The anal and oral ridges are 

 connected by two lateral cili- 

 ated bands, the whole forming 

 a continuous band, which, 

 since the mouth lies in the 

 centre of it (fig. 255), may be 

 regarded as a ring completely 

 surrounding the body behind 

 the mouth, or more naturally 

 as a longitudinal ring. 



The bilateral Auricularia 

 is developed from a slightly 

 elongated gastrula with an uniform covering of cilia. The 

 gastrula becomes flattened on the oral side. At the same time 

 the cilia become specially developed on the oral and anal ridges, 

 and then on the remainder of the ciliated ring, while they are 



IM<:. 254. A. THE LARVA OF A HOLO- 

 THUROID. B. THE LARVA OF AN ASTER- 

 OID. 



m. mouth; st. stomach; a. anus; l.c- 

 primitive longitudinal ciliated band; pr.c. 

 pra>oral ciliated band. 



m 



FIG. 255. DIAGRAMMATIC FIGURES REPRESENTING THE EVOLUTION OF AN 

 AURICULARIA FROM THE SIMPLEST ECHINODERM LARVAL FORM. (Copied from 

 Miiller.) 



The black line represents the ciliated ridge. The shaded part is the oral side of 

 the ring, the clear part the aboral side. 



in. mouth; an. anus. 



simultaneously obliterated elsewhere ; and so a complete Auricu- 

 laria is developed. The water-vascular ring in the fully-developed 

 larva has already considerably advanced in the growth round the 

 oesophagus (fig. 256 w.v.r}. 



Most Holothurian larvae, in their transformation from the 

 bilateral Auricularia form to the radial form of the adult, pass 

 through a stage in which the cilia form a number of transverse 



