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 



,+wr 



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

 PENTRACRINOID ANTEDON LARVA WITH CLOSED VESTIBULE. 



(From Carpenter ; after Gotte.) 



ae. epithelium of oral vestibule; ;//. mouth; al. mesenteron; an. rudiment of 

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

 left (oral) peritoneal sack; wr. water-vascular ring; /. 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., and 

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



