550 



CRINOIDEA. 



the ventral side (fig. 250 B). It becomes (fig. 250 C) divided into three 

 chambers, of which the two hindermost (d and c) form the stomach and 

 intestine ; while the anterior forms the oesophagus, and gives rise to the 



FIG. 250. THREE SIDE VIEWS OF EARLY STAGES IN THE DEVELOPMENT OF 



STRONGYLOCENTRUS. (From Agassiz.) 



a, anus (blastopore) ; d. stomach ; o. oesophagus ; c . rectum ; w. vaso-peritoneal 

 vesicle ; v. ciliated ridge ; r. calcareous rod. 



vaso-peritoneal vesicles. These latter appear as a pair of outgrowths 

 (fig. 251), but become constricted off as a single two-horned vesicle, which 

 subsequently divides into two. The left of these 

 is eventually divided, as in Asteroids, into a 

 peritoneal and water-vascular sack, while the 

 right forms the right peritoneal sack. An oral 

 invagination on the flattened ventral side meets 

 the mesenteron after its separation from the 

 vaso-peritoneal vesicle. The larval anus per- 

 sists, as also does the larval mouth, but owing 

 to the manner in which the water-vascular 

 rosette is established the larval oesophagus ap- 

 pears to be absorbed, and to be replaced by a 

 fresh oesophagus. 



Crinoidea. Antedon, the only Crinoid 

 so far studied (Gotte, No. 549), presents some 

 not inconsiderable variations from the usual 

 Echinoderm type. The blastopore is placed on 

 the somewhat flattened side of the oval blasto- 

 sphere, and not, as is usual, at the hinder end. 



The blastopore completely closes, and is not converted into the perma- 

 nent 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 



FIG. -251. DORSO-VEN- 

 TRAL VIEW OF AN EARLY 

 LARVA OF STRONGYLOCEN- 

 TRUS. (From Agassiz.) 



a. anus ; d. stomach ; o. 

 oesophagus ; w. vaso-perito- 

 neal vesicle; r. calcareous 

 rod. 



