S2 THE LEECH 



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 the epidermis, and 

 lie between the inner ends of the epidermal cells. 

 It is by means of this cutaneous capillary system 

 that respiration is effected. 



0. 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 longitudinally, 

 and a central medullary portion composed of granular pro- 

 toplasm and containing the nucleus. The cortical layer is 

 so sharply marked off from the medullary portion 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 



which encircle the body transversely. The layer 

 itself is not much thicker than the epidermis, but 

 the transverse muscles of the dermis should, per- • 

 haps, be grouped with it. 



2. The middle or oblique layer of muscles is more deeply 



placed, and is separated from the circular layer by a 

 stratum of connective tissue. The fibres run obliquely 

 round the body, 



3. The inner or longitudinal layer of muscles lies im- 



mediately within the oblique layer, and is by far the 

 thickest of the three layers. 



The fibres run longitudinally, and are therefore 

 seen in transverse section. They are arranged in 

 bundles, and have a very characteristic appearance, 

 looking, for the reason noticed above, like rings of 



