REPRODUCTIVE ORGANS. 65 



2. The lateral tubes are large and have distinct muscular 



walls. 



3. The capillaries are very thin-walled tubes, the distribu- 



tion of which can easily be followed owing to the 

 contained blood. The intercellular plexuses of the 

 epidermis and of the nephridia should be specially 

 noticed. 



4. The botryoidal tissue consists of a network of vessels, 



, the channels of which are irregular in width and 

 the walls formed of large granular pigmented cells 

 which project as irregular rounded swellings. 



These curiously swollen vessels occur in great 

 abundance around the crop, immediately within the 

 longitudinal muscle-layer. They are in free com- 

 munication with the ordinary blood-capillaries. 



. The Reproductive Organs. 



1. The testes are seen, if the section happens to pass 



through them, as spherical sacs lying at the sides 

 of the ventral sinus, and containing groups of 

 spermatozoa in various stages of development. 



2. The vasa deferentia lie to the outer sides of the testes : 



each is a thick-walled tube, with a small central 

 lumen. 



Fig. 24. 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. 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. G, cauliflower head of nephridium. 

 I, vesicle of nephridium. K, external aperture of nephridium. |_V, lateral 

 tube. N, nerve cord, o, testis with developing spermatozoa. P, vas deferens. 

 PM, posterior limb of main lobe of nephridium. PS, peri-nephrostomial sinus. 

 R, section of nephridium. S, intracellular ductules of nephridium. TL, testis- 

 lobe of nephridium. VS, ventral sinus. X, botryoidal tissue. Y, dorco- 

 ventral muscles. 



