PLATE XLIII 



Cephalodiscus nigrescens, Lankester 



Fig. I . Median longitudinal section of an embryo showing the ventral 

 thickening and the inner mass of yolk granules. 



Fig. 2. Longitudinal section of a young larva showing the formation of 

 the coelomic cavities. 



Fig. 3. Longitudinal section of a larva of a later stage, in which the 

 alimentary canal is a straight tube with the anus situated at the posterior 

 end. 



Fig. 4. Longitudinal section of a very young C nigrescens. 



a, proboscis; al, alimentary canal; an, anus; arch, archenteron; b, brain; 

 cc, collar cavity; cp, undivided coelomic pouch connected with the 

 archenteron; d, notochord; m, mouth; pc, proboscis cavity; pd, dorsal 

 pharyngeal diverticulum; pg, pigment granules; ra, rudiment of the 

 first pair of arms; rb, refractive beads; tc, trunk coelom; vp, ventral 

 prolongation of the trunk to give rise to the stolon ; vt, ventral thickening ; 

 yg, yolk granules. 



