V TRANSVERSE SECTION 321 



Stopping the ends of it with corks, boring a hole in each 

 cork, and then inserting 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 

 communication with the outside. A transverse section of 

 the body has, therefore, the general character of two con- 

 centric circles. 



It will be remembered that a transverse section of Hydra 

 has the character of two concentric circles, formed re- 

 spectively of ectoderm and endoderm (Fig. 74, p. 293), the 

 two layers being, however, only separated by the thin 

 mesogloea. At first sight then, it seems as if we might 

 compare the earthworm to a Hydra in which the ecto- 

 derm 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 com- 

 parison would only express part of the truth. 



A thin transverse section (Fig.. 78) shows the body-wall of 

 the earthworm to consist of four distinct layers. Outside is 

 a thin transparent cuticle (cut) showing 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 oi muscle-fibres hy 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 a 



Pract. Zool. V 



