MUSEUM OF COMPARATIVE ZOOLOGY. 83 
The portion of the alimentary canal first to appear — the stomodzum 
—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 stomodzum arises behind the 
stomach, and, extending through the cephalothorax, opens widely, by 
a bell-shaped expansion, into the yolk of the abdomen (Pl. XII. fig. 
78). This post-gastric portion of the canal was evidently overlooked by 
Balfour, as he (1. ¢., p. 187) states that he was unable to find “any trace 
of an anterior part of the mesenteron adjoining the stomodzeum.” 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 wanting. 
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 csophagus. 
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. XI. fig. 70, mu.). From the sucking stomach three distinct muscles 
extend to the body wall: a vertical muscle (mw. 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 (mw. dat.), 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 (Pl. IX. fig. 62; Pl. 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). 
Tam in doubt concerning the origin of the post-gastric tube already 
alluded to. Its anterior end, which lies just beneath the stomach 
(Pl. XII. fig. 78), is rounded and plugged with cells, and I have been 
able to trace an enveloping layer of mesodermic elements nearly across 
