TRANSVERSE SECTIONS. 55 



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 longitudinally, 

 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, 



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 connectivetissue. The fibres run obliquely 

 round the body. 



Fig. 22. — Hirudo medioinalis. A diagiammatic transverse section 

 through the middle of the body. On the left side of the figure the 

 whole of a nephridium is drawn, in order to show its relations : the 

 anterior surface of the section is towards the spectator, and the 

 two limbs of the main lobe of the nephridium are slightly displaced, 

 being represented as inner and outer, instead of anterior and pos- 

 terior respectively, (a. m. m.) 



A, crop. AL, apical lobe of nephridium. AM. anterior limb of maiu 

 lobe of nephridium. B, right diverticulum of crop. C, left diverticulum of 

 crop. CIT, cuticle. D,, epidermis. DS, dorsal sinus, l!/, layer of circular 

 and oblique muscles. F, longitudinal muscle-layer. G, cauliflower head of 

 nephridium. I, vesicle of nephridium. K, external aperture of nephridium. 

 LV. lateral vessel. IT, nerve-cord. O, testis with developing spermatozoa. 

 P, vas deferens. PM, postei'ior limb of maiu lobe of nephridium. PS, peri- 

 uephrostomial sinus. R, section of nephridium. S, iutracellular ductules of 

 nephridium. TL, testis-lobe of nephridium. VS, ventral sinus. X, botry- 

 oidal tissue. Y, dorso-ventral muscles. . 



