550 



CEPHALOCHORDA. 



segment and arises as a transverse groove in the floor of the enteron 

 (Fig. 287, dr) specially distinct on the right side of the body and 

 running thence ventrally to the left side. This groove deepens and 

 becomes constricted off from the enteron to form an independent 

 tube, and then represents the club-shaped gland, the right, blind 

 portion of which is dilated, while the narrowed left section opens 

 externally in front of the oral aperture (Fig. 288, A 1 '). In later stages, 

 the right, blind end of this gland enters into communication with the 

 lumen of the intestine (RAY LANKESTER and WILLEY). 



ds ds 



FIG. 289. A, anterior eud of a larva somewhat older than that depicted in Fig. 288, 

 seen from the right side ; B, posterior end of the same seen from the left side, an, 

 anus ; c, larval caudal fin ; ch, chorda ; d, alimentary canal ; ds, dissepiments of the 

 side turned to the spectator ; ds', dissepiments of the other side ; Ji, ciliated band 

 (rudiment of the endostyle) ; k, club-shaped gland ; ks, gill-cleft ; m, mouth ; in#>, 

 pole-cells of the mesoderm ; mr, medullary tube ; mr', posterior end of the medullary 

 tube ; np, neuropore ; si.\ sub-intestinal vein ; w, ciliated organ (pre-oral pit). 



Immediately in front of the club-shaped gland there is a transverse 

 ciliated band (Fig. 289 A, fl) which, according to WILLEY, is the 

 first rudiment of the endostyle. 



The oral aperture (Fig. 289, m) forms on the left side of the body 

 in the region of the first segment. A disc-like thickening of the 

 ectoderm first appears, to the inner side of which the intestine 

 becomes closely applied, since, in this region, the mesoderm does not 



