VI TRANSVERSE SECTION 331 



fibres and connective-tissue, and an outer layer of large 

 yellow cells, the function of which is said to be excretory, 

 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 the Hydra and 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 Obelia (viz., the perisarc). 

 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 mesogloea. 

 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 

 sometimes found a layer of separate muscle-fibres 

 between the ectoderm and the mesoglcea, and it was 

 pointed out (p. 311) that such fibres represented a 

 rudimentary intermediate cell-layer or mesoderm. We 

 may therefore consider 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, and an inner or visceral layer. 



The parietal layer is in contact with the ectoderm or 

 deric epithelium, and with it forms the body-wall ; the 

 visceral layer is in contact with the endoderm or enteric 

 epithelium, and with it forms the enteric canal. The 



