62 THE LEECH. 



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 clitellar 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 slender ductule. 



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. 



C. The Muscles. 



The muscles of the leech consist of long fusiform cells 

 arranged either singly or in bundles. Each muscle-cell con- 

 sists of an outer cortical layer which is striated longitud'nally, 

 and a central medullary portion composed of granular pro- 

 toplasm and containing the nucleus. The cortical layer is 

 sharply marked off from the medullary portion, so that in a 

 transverse section of a muscle-cell it appears as a ring, radially 

 striated, and surrounding a central finely granular mass. 

 1. The outer or circular layer of muscles consists of fibres, 



