532 



INVERTEBEATA 



CHAP. 



into oesophagus, stomach, and intestine. This last division is 

 bent forwards, so that, as in other Echinoderm larvae, the alimentary 

 canal is bent in a curve, concave ventrally. The stomodaeum is 

 formed in the usual manner on the ventral side of the larva; it 

 joins the oesophagus after the coelom has been separated from the 

 alimentary canal. 



The general covering of cilia becomes lost as the ectoderm, cells 

 flatten out, and the cilia become restricted to ridges which form a 



coe 



FIG. 396. Two young larvae of Synapta digitata viewed from the side. (After Selenka. ) 



A, formation of the coelom. The coelomic part of the archenteron opens to the exterior by the madre- 



poric pore before it is separated from the gut-portion of the archenteron. B, formation of the mouth and 



stomodaeum. al, alimentary canal ; ap, apical thickening ; blp, blastopore ; coe, coelomic vesicle ; mp, 



madreporic pore ; stom, stomodaeum. 



folded longitudinal band. This band resembles that of the Bipinnaria 

 larva of Asteroidea in its earlier stages of development, i.e. it consists 

 of right and left longitudinal pieces and anterior and posterior cross- 

 bars, the former of which is bent backwards over the prae-oral region 

 of the larva as the prae-oral loop, and the latter of which is bent 

 forwards in front of the anus as the anal loop. An adoral band of 

 cilia is formed in the same way as has been described for the 

 Bipinnaria, Ophiopluteus, and Echinopluteus. 



The Holothuroid larva differs from the Bipinnaria in two points : 

 first, the prae-oral loop, although almost, is never quite constricted 

 from the rest of the band, so that the longitudinal band of cilia 



