TEANSVEKSE 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 Integument. 



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



tureless elastic cuticle, which is formed by the under- 

 lying epidermis, and is shed and renewed at frequent 

 intervals during life. 



2. The epidermis consists of a single layer of columnar 



nucleated cells, wider at their outer than 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 

 exterior 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 



Fm. 15. 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 

 posterior respectively. (A. M. M.) 



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

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

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

 oblique muscles. F, longitudinal muscle-layer. Gr, cauliflower head of nephri- 

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

 lateral vessel. N, nerve-cord. O, testis with developing spermatozoa. P, 

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

 nephrpstomial sinus. R, section of nephridium. S, intrafeellular ductules of 

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

 oidal tissue. Y, radial muscles. 



E2 



