TRANSVERSE SECTIONS 53 



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 

 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 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 

 various sizes packed together in groups, and usually 



Pig. 22. — Hirudo medicinalis. A diagrammatic 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. AXj, apical lobe of nephridium. AM, anterior limb of main 

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

 crop. OTT, cuticle. D, epidermis. DS, dorsal sinus. E, layer of circular 

 and oblique muscles. P, longitudinal muscle-layer. Gt, cauliflower head of 

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

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

 P, vas deferens. PM, posterior limb of main lohe of nephridium. PS, peri- 

 nephrostomial sinus, R, section of nephridium. S, intracellular ductules of 

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

 oidal tissue. Y, dorso-ventral muscles. 



