V TRANSVERSE SECTION 323 



connective tissue, and an outer layer of large yelloni cells, 

 the function of which is not thoroughly understood, and 

 which correspond to a special development of the ccelomic 

 epithelium covering the visceral] layer of the peritoneal 

 membrane which invests the intestine. 



We are now in a better position to compare the trans- 

 verse section of Hydra and of the earthworm. The 

 epiderm of the earthworm being the outermost cell-layer 

 is to be compared with the ectoderm of Hydra, and its 

 cuticle with the layer of the same name which, though 

 absent in Hydra, is present in the stem of hydroid polypes, 

 such as Bougainvillea (p. 306). The enteric epithelium 

 of the earthworm, bounding as it does the digestive 

 cavity, is clearly comparable with the endoderm of 

 Hydra. So that we have the double layer of muscle- 

 fibres and the two layers of peritoneum not represented 

 in Hydra, in which their position is occupied merely 

 by the mesoglcea. The muscle-fibres are not of the striped 

 kind, like those in the corresponding position in the frog 

 (p. 112). 



But it will be remembered that in Medusae there is some- 

 times found a layer of separate muscle-fibres between the 

 ectoderm and the mesoglcea, and it was pointed out 

 (p. 304) that such fibres represented a rudimentary inter- 

 mediate cell-layer or mesoderm. We may therefore con- 

 sider the muscular layer and the peritoneum of the 

 earthworm as mesoderm, and we may say that in this 

 animal, as in the frog (p. 203 and Fig. 65), the mesoderm 

 is divisible into an outer or parietal layer, an inner or 

 visceral layer. 



The parietal layer is in contact with the ectoderm or deric 

 epithehum, and with it forms the body-wall ; the visceral 

 layer is in contact with the endoderm or enteric epithelium, 



y 2 



