THE ALIMENTARY CANAL. 



197 



also obliterated in the further course of development, and the 

 intestine then runs straight back. The stomodaeum gives rise in 

 Peripatus to the muscular oesophageal swelling (pharynx), meso- 

 dermal tissue also contributing to its formation. The external 

 changes in the mouth have already been described in connection 

 with the external form of the body (Fig. 94, p. 1.85). The growth 

 of the embryo produces similar changes at the posterior end of the 

 intestine. 



A 



B. 



C. 



Fig. 98.— Median longitudinal sections through embryos of P. capensis at various ag>\s (after 

 Sedgwick), an, anus; di, anterior entoderraal diverticulum: ent, entoderm; m, mouth; 

 st, stomodaeum. 



In the American species of Peripetias, the intestine even at it^ first appearance 

 differs from that of P. capensis, as no elongation of the blastopore occurs in these 

 forms (v. Kennel). The rndiment of the enteron, which is completely closed 

 to the exterior and has been produced by the ingrowth of cells (Figs. SO, p. 171, 

 and SI, p. 173), is here sac-shaped. As the embryo lengthens, the enteron 

 also extends in the form of a tube. Its connection with the ectoderm is brought 

 about through the fusion of the entoderm with the ectoderm, an invagination 

 of the latter at this point forming the oral aperture. The mouth arises ventrally 

 on the boundary between the head and tin- trunk, and the anus in front of the 

 blastopore (Fig. 89 A). It has already been pointed out (p. 17S) that these 

 two apertures occupy the same positions as in /'. capi nsis, and that they perhaps 



