274 POLYGORDIUS iess. 



{M. PI), having the form of long flat spindles (Fig. 69, p. 

 284, M. Fl.) exhibiting a delicate longitudinal striation and 

 covered on their free services with a fine network of proto- 

 plasm containing scattered nuclei. Each plate is arranged 

 longitudinally, extending through several segments, and with 

 its short axis perpendicular to the surface of the body (Fig. 

 70, M. PL). It is by the contraction of the muscle-plates 

 that the movements of the body, which resemble those of 

 an earthworm, are produced. Finally, within the muscular 

 layer and lining the coelome is a very thin layer of cells, the 

 calomic epithelium ( Cixl. Eptlim). 



A transverse section of the enteric canal shows only two 

 layers. The inner consists of elongated cells {£nf. Epthm) 

 fringed on their inner or free surfaces with cilia : these con- 

 stitute the enteric epithelium. Outside these is a very thin 

 layer of flattened cells {Coil. Epthm) bounding the coelome, 

 and hence called, like the innermost layer of the body-wall, 

 ccelomic epithelium. We have, therefore, to distinguish 

 two layers of coelomic epithelium, ^n outer or parietal layer 

 {Ccel. Epthm.) which lines the body-wall, and an inner or vis- 

 ceral layer {Ccel. Epthm') which invests the enteric canal 



We are now in a better position to compare the transverse 

 sections of Hydra and of Polygordius (Fig. 55, a', and Fig. 

 67, c). The deric epithelium of Polygordius 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. 238). The enteric 

 epithelium of Polygordius, bounding as it does the digestive 

 cavity, is clearly comparable with the endoderm of Hydra. 

 So that we have the layer of muscle-plates and the two layers 

 of coelomic epithelium not represented in Hydra, in which 

 their position is occupied merely by the mesogloea. 



