Ill-] THE EARTHWORM. 243 



state or in concretionary masses sometimes reaching a 

 diameter of 1 mittim. Such being the case they are termed 

 calciferous glands, but the purpose served by them is as yet 

 not fully understood. 



The body-wall is invested in a single layer of elongated 

 epidermal cells, many of which become converted into 

 small unicellular cutaneous glands. This investing epi- 

 thelium of the body and the cuticle which it secretes (vide 

 supra) are continuous respectively, at the mouth and anus, 

 with the epithelial lining of the alimentary canal and its 

 cuticular product. The greater mass of the body-wall is 

 however composed of two sets of muscular fibres, an ex- 

 ternal circular set and an internal longitudinal one; these 

 also pass, at opposite ends of the body, into an internal 

 circular and an external longitudinal series of fibres entering 

 into the composition of the wall of the digestive tube. The 

 two walls are widely separated by a spacious body-cavity, 

 the alimentary canal being suspended by a series of trans- 

 verse mesenteric septa, instead of by a median longitudinal 

 fold as in the Frog. These septa are metamerically ar- 

 ranged, one for each somitic constriction ; circular muscular 

 fibres are developed within them and radial ones pass across 

 them from body-wall to wall of alimentary canal. The 

 body-wall is lined internally by a peritoneum, clad in 

 tessellated epithelium, which can be traced on to both the 

 mesenteric septa and the free surfaces of the various organs 

 contained within the body-cavity. It is from this layer that 

 the chloragogen cells aforenamed take their origin. The 

 walls of the body and alimentary canal are thus both seen 

 to consist of at least three clearly defined layers; a pro- 

 tective cuticular, an epitheloid, and a mechanical muscular 

 one respectively. These layers are continuous at both 

 mouth and anus, and the order of succession is the same 



16 — 2 



