5 62 



OPHIUROID PLUTEUS. 



of the disc. At a later stage the anus is formed on the dorsal side of an arm 

 opposite the mouth. The stomach is surrounded by a water-vascular ring, 

 from which the madreporic canal passes to the dorsal surface, but does not 

 open. At a later stage the embryonic mouth and anus vanish, to be replaced 

 by a permanent mouth and anus in the normal positions. 



A third, and in some respects very curious, form is a worm like larva of 

 Miiller, which is without bands of cilia. The dorsal surface of the youngest 

 larva is divided by transverse constrictions into five segments. On the 

 under side of the first of these is a five-lobed disc, each lobe being provided 

 with a pair of tube feet. 



At a later period only three segments are visible on the dorsal surface, 

 but the ventral surface has assumed a pentagonal aspect. The later stages 

 are not known. 



Ophiuroidea. The full-grown larva of the Ophiuroids is 

 known as a Pluteus. It commences with the usual more or less 

 spherical form ; from this it passes to a form closely resembling 



A 



FIG. 261. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF AN OPHIU- 

 ROID PLUTEUS FROM A SIMPLE ECHINODERM LARVA. (Copied from Miiller.) The 

 calcareous skeleton is not represented. 



/;/. mouth; an. anus; d. anterior arms; d '. lateral arms; e'. posterior arms; g . 

 anterolateral arms. 



that of Auricularia with a rounded dorsal surface, and a flattened 

 ventral one. Soon however it becomes distinguished by the 

 growth of a post-anal lobe and the absence of a prae-oral lobe 

 (fig. 261 B). The post-anal lobe forms the somewhat rounded 

 apex of the body. In front of the mouth, and between the 

 mouth and anus, arise the anal and oral ciliated ridges, which 

 soon become continued into a single longitudinal ciliated ring. 

 At the same time the body becomes prolonged into a series of 



