Crinoidea. Antedon, the only Crinoid so far studied (Götte, No. [549]), presents some not inconsiderable variations from the usual Echinoderm type. The blastopore is placed on the somewhat flattened side of the oval blastosphere, and not, as is usual, at the hinder end.
Fig. 251. Dorso-ventral view of an early larva of Strongylocentrus. (From Agassiz.)
a. anus; d. stomach; o. œsophagus; w. vaso-peritoneal vesicle; r. calcareous rod.
The blastopore completely closes, and is not converted into the permanent anus. The archenteron gives rise to the epithelioid lining of both body cavity and water-vascular system. These parts do not, however, appear as a single or paired outgrowth from the archenteron, but as three distinct outgrowths which are not formed contemporaneously. Two of them are first formed, and become the future body cavity; but their lumens remain distinct. Originally appearing as lateral outgrowths, the right one assumes a dorsal position and sends a prolongation into the stalk ([fig. 252], rp´), and the left one assumes first a ventral, and then an oral position ([fig. 252], lp).
Fig. 252. Longitudinal section through an Antedon larva. (From Carpenter; after Götte.)
al. mesenteron; wv. water-vascular ring; lp. left (oral) peritoneal vesicle; rp. right peritoneal vesicle; rp´. continuation of right vesicle into the stalk; st. stalk.