552 



CRINOIDEA. 



cavity. In the adult 1 the oral section of the larval body cavity becomes the 

 ventral part of the circumvisceral division of the body cavity, and the 

 subtentacular canals of the arms and disc ; while the aboral section becomes 

 the dorsal part of the circumvisceral division of the body cavity, the cceliac 

 canals of the arms, and the cavity of the centro-dorsal piece. The primitive 



FIG. 253. LONGITUDINAL SECTION THROUGH THE CALYX OF AN ADVANCED 

 PENTRACRINOID ANTEDON LARVA WITH CLOSED VESTIBULE. 



(From Carpenter ; after Gotte.) 



at. epithelium of oral vestibule ; m. mouth ; al. mesenteron ; an. rudiment of 

 permanent anus; Ip. posterior part of left (oral) peritoneal sack; //'. anterior part of 

 left (oral) peritoneal sack; lur. water-vascular ring; t. tentacle; mt. mesentery; 

 rp. right peritoneal sack; rp' '. continuation of right peritoneal sack into the stalk; 

 r. roof of tentacular vestibule. 



distinction between the sections of the larval body cavity becomes to a large 

 extent obliterated, while the axial and intervisceral sections of the body 

 cavity of the adult are late developments. 



The more important points in the development indicated in 

 the preceding pages are as follows : 



(i) The blastosphere is usually elongated in the direction 

 of the axis of invagination, but in Comatula it is elongated 

 transversely to this axis. 



1 Vide P. H. Carpenter, " On the genus Actinometra." Linnean Trans., 2nd 

 Series, Zoology, Vol. n., Part I., 1879. 



