TRANSVERSE SECTIONS 51 



A. Shape. 



A transverse section of the body of a leech is oval in out- 

 line, the transverse diameter being the greater, and the ventral 

 surface being flatter than the dorsal. 



B. The Integniment. 



1. The cuticle. The most superficial layer is a thin struc- 



tureless elastic cuticle, which is secreted by the under- 

 lying epidermis, and is shed and renewed at frequent 

 intervals during Hfe. 



2. The epidermis consists of a single layer of columnar 



nucleated cells, wider at their outer than at their 

 inner ends. 



a. The epidermal glands. Certain of the epidermal 

 cells are modified to form unicellular glands. 

 These may remain in the epidermal layer as 

 mucous glands ; or they may sink down into the 

 muscular layers, as in the case of the salivary 

 glands, and of the elitellar glands of the fifth to 

 the seventh somites, which latter secrete the 

 cocoon in which the eggs are laid. 



In all cases each cell is a complete gland in 

 itself, and preserves its communication with the 

 surface by means of a long tubular stalk. 



3. The dermis is a layer of some thickness lying between 



the epidermis and the muscular coats. It consists of 

 a jelly-like matrix containing numerous branched 

 corpuscles. It is traversed by pigmented fibres, 

 which branch and anastomose very freely, and pene- 

 trate between the epidermal cells : to these the colour 

 of the skin is due. There are also in the dermis 

 irregularly arranged muscle-fibres, chiefly transverse 

 in direction, and a very abundant capillary plexus, 

 the branches of which penetrate between the inner 

 ends of the epidermal cells. It is by means of this 

 cutaneous capillary system that respiration is effected. 



B 2 



