TRANSVERSE SECTIONS 73 



lying between these are extremely slender. In the 

 clitellum the epidermis is greatly thickened, and 

 blood-vessels penetrate between the 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. Setse in various 

 stages of development may be seen withia 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 side^ of a series of radial longitudinal sheets, and 

 having a feather-Uke appearance in transverse section. 



D. The Coelom, or Body-cavity. 



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

 body- wall. It is Hned by a thin peritoneal epithelium. 



E. The Intestine. 



This occupies the middle of each section. Its diameter 

 is about half that of the body of the worm. 



1. The typhlosole is a fold of the dorsal wall extending 



into the intestine from above, narrowing its cavity, 

 and making it crescentic in transverse section. 



2. The epithelium lining the intestine consists of a single 



layer of columnar ciliated ceUs. 

 8. The muscular layer of the intestiaal wall is thin. The 

 fibres, which are nearly all transverse in direction, 

 are imbedded in a connective-tissue layer. 



