468 



OPHIUROID PL UTE US. 



A second abnormal type of development is presented by the embryo 

 of Pteraster miliaris, as described by Koren and Danielssen 1 . The larvfe 

 to the number of eight to twenty develop in a peculiar pouch on the dorsal 

 surface of the body. The early stages are not known, but in the later 

 ones the whole body assumes a pentagonal appearance with a mouth at 

 one edge 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 that of Auricu- 

 laria 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 prse-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 longitu- 

 dinal ciliated ring. At the same time the body becomes prolonged 



FIG. 261. DIAGRAMMATIC FIGURES SHEWING THE EVOLUTION OF AN OPHIUROID 

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

 careous skeleton is not represented. 

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



<j ' . anterolateral arms. 



1 The following statements are taken from the abstract in Bronn's Thierreichs. 



