vi TRANSVERSE SECTION 329 



with corks, boring a hole in each cork, and then insert- 

 ing through the holes a narrow tube of the same length 

 as the wide one. The outer tube would represent the 

 body-wall, the inner the enteric canal, and the cylindrical 

 space between the two the coelome. The inner tube 

 would communicate with the exterior by each of its ends, 

 representing respectively mouth and anus ; the space 

 between the two tubes, on the other hand, would have 

 no such communication with the outside. A transverse 

 section of the body has, therefore, the general character 

 of two concentric circles. 



It will be remembered that a transverse section 

 of Hydra has the character of two concentric circles, 

 formed respectively of ectoderm and endoderm (Fig. 76, 

 p. 298), the two layers being, however, only separated 

 by the thin mesoglcea. At first sight, then, it seems as 

 if we might compare the earthworm to a Hydra in which 

 the ectoderm and endoderm, instead of being in contact, 

 were separated by a wide interval ; we should then 

 compare the body-wall of the earthworm with the 

 ectoderm of Hydra, and its enteric canal with the 

 endoderm. But this comparison would only express 

 part of the truth. 



A thin transverse section (Fig. 81) shows the body- 

 wall of the earthworm to consist of several distinct 

 layers. Outside is a thin transparent cuticle (cut) show- 

 ing no structure beyond a series of intersecting oblique 

 lines. Next comes a layer of epithelium, the epiderm 

 or deric epithelium (epid). Within this is a very thin 

 connective-tissue layer representing the derm (p. 128), 

 and a double layer of muscle-fibres by means of which 

 the movements of the body are produced an outer, in 

 which the fibres extend transversely round the body 

 (circ. mus), and a much thicker inner layer consisting of. 

 longitudinal fibres, in section arranged like the barbs of 



