MUSEUM OF COxMPARATIVE ZOOLOGY. 83 



The portion of the alimentary canal first to appear — the stomodseum 

 — arises as an invagination of ectoderm just before the beginning of 

 the third period of development, and therefore after the establishment 

 of a mesodermic layer in the region in which the invagination occurs. 

 As already stated, it grows rapidly during the third period, and in the 

 fourth period it acquires certain muscular attachments, developed out 

 of mesodermic cells. After the reversion of the embryo is completed, 

 a tube of about the same calibre as the stomodsum arises behind the 

 stomach, and, extending through the cephalothorax, opens widely, by 

 a bell-shaped expansion, into the yolk of the abdomen (PI. XII. fig. 

 78). This post-gastric portion of the canal .was evidently overlooked by 

 Balfour, as he (1. c, p. 187) states that he was unable to find "any trace 

 of an anterior part of the mesenteron adjoining the stomodseum." An- 

 teriorly it apparently does not open into the sucking stomach during 

 embryonic stages, but is so plugged with cells that its relations are 

 obscured. 



At the time of hatching the intestinal tract is still incomplete, the 

 epithelial wall of the mesenteron being largely or altogether w^anting. 

 There may be distinguished in the anterior portion of the tract the fol- 

 lowing parts : pharynx, oesophagus, sucking stomach, and post-gastric 

 tube. 



The pharynx passes from the mouth obliquely upwards and backwards, 

 and, turning at nearly a right angle, is continued into the oesophagus. 

 The latter is of uniform calibre and extends backwards with a slightly 

 downward curve, terminating in the enlarged sucking stomach. A 

 muscle arising from the dorsal wall of the cephalothorax just in front of 

 the anterior margin of the brain, is inserted at the angle of the pharynx 

 (PL XL fig. 10, mu.). From the sucking stomach three distinct muscles 

 extend to the body wall : a vertical muscle {imt. vrt.), lying in the sagittal 

 plane and extending downward from the cephalothorax a little behind 

 the brain to be inserted along the dorsal wall of the stomach ; a pair of 

 lateral muscles (mu. lat.), which arise from the sternal plate and ascend 

 obliquely towards the sagittal plane to be inserted into the lower half of 

 the lateral walls of the stomach (PI. IX. fig. 62 ; PI. XI. fig. 70). A 

 few fibres arising with the vertical muscle join the fibres of the lateral 

 muscles without having a distinct attachment to the stomach (Fig. 62). 



I am in doubt concerning the origin of the post-gastric tube already 

 alluded to. Its anterior end, which lies just beneath the stomach 

 (PI. XII. fig. 78), is rounded and plugged with cells, and I have been 

 able to trace an enveloping layer of mesodermic elements nearly across 



