272 POLYGORDIUS 



The outer tube would represent the body-wall, the inner the 

 enteric canal, and the cylindrical space between the two the 

 ccelome. The inner tube would communicate with the ex- 

 terior 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. 



Polygordius is the first example we have studied of a 

 cozlomate animal : one in which there is a definite body- 

 cavity separating from one another the body-wall and the 

 enteric canal, and in which therefore a transverse section of 

 the body has the general character of two concentric circles 

 (Fig. 69, c). 



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. 55, p. 238, A'), 

 the two layers being however in contact or only separated 

 by the thin mesoglsea. At first sight then it seems as if we 

 might compare Polygordius 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 Polygordius 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 shows the body-wall of Poly- 

 gordius to consist of four distinct layers. Outside is a thin 

 transparent cuticle (Fig. 69, c, and Fig. 72, A, cu) showing 

 no structure beyond a delicate striation. Next comes a 

 layer of epithelial cells (Der. Epthm^, their long axis at 

 right angles to the surface of the body, and the boundaries 

 between them very indistinct, so as to give the whole layer 

 the character of a sheet of protoplasm with regularly dis- 

 posed nuclei : this is the deric epithelium or epidermis. 

 Within it is a rather thick layer of muscle plates (M. PL] 



