458 



ECHJNOIDEA. 



to be at first solid, and only subsequently to acquire a cavity the perma- 

 nent body cavity. The anus naturally disappears, since it is absent in the 



C 



B 



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



STRONGYLOCENTRUS. (From Agassiz. ) 



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

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



adult. In the viviparous type the first stages are imperfectly known, but 

 it appears that the blastopore vanishes before the appearance of the mouth. 

 The development of the vaso-peritoneal bodies takes place as in the 

 free-swimming larva?. 



Echinoidea. In the Echinoidea (Agassiz, No. 542, Selenka, No. 564) 

 there is a regular segmentation and the normal invagination (fig. 250 A). 

 The amoeboid mesoblast cells arise as two laterally placed masses, and give 

 rise to the usual parts. Tlie archenteron grows forwards and bends to- 

 wards the ventral side (fig. 250 B). It becomes (tig. 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 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. 



Cl'ilioidea. Antedon, the only Crinoid so 

 far studied (Gotte, No. 549), presents some not 

 inconsiderable variations from the usual Echi- 

 noderm type. The blastopore is placed on the 

 somewhat flattened side of the oval blasto- 

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



Fl. 251. DOHSO-VENTBAL 



VIEW OF AN EARLY LARVA OF 



STRONGYLOSOJIA. (From Agas- 

 siz.) 



. anus ; d. stomach ; o. 

 oesophagus; w. vase-perito- 

 neal vesicle; r. calcareous rod. 



