XIV 



CHAETOGNATHA 



431 



ably in length, so that the anterior convex end bears but a small pro- 

 portion to the total length. The central division of the archeiiteron 

 becomes closed off from the two lateral divisions behind, by the 

 approximation and union of the inner walls of its folds, and in this 

 way the gut is separated from the coelomic sacs. 



The front portions of the coelomic sacs, lying at the sides of the 

 stomodaeum, become separated from the rest and form a pair of round 

 thick-walled head-cavities. The cavities are soon obliterated, but 

 their walls give rise to the muscles which move the curved setae 

 situated near the mouth, which give the name to the phylum. 



The masses of mesoderm which represent the head -cavities 



.c 



FIG. 340. Optical section of older embryo of Sagitta bipunctata, showing the formation of 

 the "head -cavities " and of the stomodaeum. (After Doncaster. ) 



coe, coelomic sac ; ent, enteron ; g\ mother cell of ovary ; 02, mother cell of the testis ; h.c, head- 

 cavity ; stom, stomodaeum. 



touch each other dorsally, but ventral ly are separated by the 

 stomodaeum, so that they resemble a horse shoe in shape. Traced 

 backwards these masses of head-inesoderm are seen to overlap the 

 front ends of the hinder portions of the coelomic sacs which are nearer 

 the middle line. 



The blastopore closes completely ; its last trace is on the ventral 

 surface, far back but not quite at the posterior end, and at the same 

 time an ectodermic invagination makes its appearance at the anterior 

 end of the embryo ; this is the stomodaeum, which grows backwards 

 and breaks through into the gut. 



The coelomic sacs continue for a short time to open into a common 

 sac behind, which occupies about one-third the length of the whole 

 embryo, but soon this single sac becomes divided into right and left 

 halves by the backward growth of a septum. This septum is formed 



