86 THE EARTHWORM. 



clitellum the epidermis is greatly thickened. Blood- 

 vessels penetrate between the epidermal cells. 



3. The dermis is an exceedingly thin layer immediately 



beneath the epidermis. 



4. The setae are lodged in sacs, which are invaginations of 



the skin extending deep into the body, through both 

 the muscular layers. They are cuticular structures, 

 and are moved by special muscles. Setae in various 

 stages of development may be seen within the sacs. 



C. The Muscular Layers of the Body-wall. 



1. The external layer, which is rather thicker than the 



epidermis, consists of transverse fibres running 

 round the body : it is completely interrupted at the 

 grooves between the segments. The connective 

 tissue between the muscle fibres is pigmented 

 slightly ; in the dorsal wall of the oesophageal 

 region this pigmentation is very marked. 



2. The internal layer is very much thicker than the ex- 



ternal. It consists of longitudinal fibres, arranged 

 on the sides of a series of radial longitudinal sheets, 

 and having a feather-like appearance in transverse 

 section. 



D. The Coelom, or Body-cavity. 



This is the space around the intestine, between it and the 

 body-wall. On its outer surface it is lined by a thin peri- 

 toneal epithelium, and contains a colourless fluid, with numer- 

 ous amoeboid corpuscles. 



1. The peritoneal 'yellow' cells He outside the intestine 

 and surround the vessels which return the blood from 

 the intestine to the dorsal vessel. They are elon- 

 gated granular cells of large size, and generally 

 arranged radially to the surface of the intestine. 

 They are very abundant in the cavity of the typhlo- 

 sole, which they almost completely fill. 



