280 



ECHINODEKMATA. 



or less pear-shaped larva, in which a slightly arched dorsal, two 

 symmetrical lateral, and a saddle-shaped ventral surface can be dis- 

 tinguished. The cilia which are concentrated upon the raised edge 

 of the ventral depression give rise to a continuous ciliated band 

 which serves as a locomotive apparatus. [This band first appears as 

 two separate ciliated ridges placed transversely, one in front of, and 

 the other behind the mouth (fig. 224, 3). These soon become con- 

 nected laterally.] The alimentary canal, which has now acquired an 

 anterior opening, the mouth, consists of three portions, the eso- 

 phagus, the stomach, and the intestine. The wide mouth leading 

 into the oesophagus is situated within the band of cilia on the 

 ventral surface ; the anus is also ventral, but external to the ciliated 

 band in the region of the posterior pole. Before the appearance 



FIG. 22!. Larval development of Aeferacanthion beryliiuis (after A. Agassiz) (for earlier 

 stages see fig. 103). 1, stage where the mouth (O) has just appeared, represented in 

 profile ; A, blastopore (anus) ; D, intestine ; Vp, vaso-peritoneal sac. 2, Somewhat older 

 stage in surface view with two separated vaso-peritoneal sacs. 3, Later stage, from the 

 ventral side, with two transverse ciliated ridges (W) ; the sac on the left side has an 

 excretory pore. 4, Young Bipinnaria with double band of cilia (IV). 



of the mouth, another organ is separated from the alimentary canal : 

 this is a sac-like ciliated tube, which opens to the exterior by a pore 

 on the dorsal surface, and represents the first commencement of the 

 ambulacral system. A second organ, which also has its origin from 

 the rudimentary digestive canal, consists of the disc-shaped lateral 

 sacs (fig. 224), from the walls of which the peritoneal lining of the 

 body cavity is produced. 



With their progressive development the larvse of the Sea-urchin, 

 the Starfish, and the Holothurian diverge more and more widely from 

 one another. The raised edge of the depression just mentioned, with 

 its band of cilia, oecomes bent and prolonged into processes (fig. 



