MEDICINAL LEECH. 



45. MEDICINAL LEECH (Hirudo mcdicinalis), 



Dissected so as to show its reproductive and segmental organs or nephridia in situ. 



A BLACK bristle has been passed into the pharynx through the nerve- 

 ring, and the ventral chain of ganglia is visible throughout the greater part 

 of its extent. In the middle line, covering the sixth ganglion of the ventral 

 chain, is seen a globular body, projecting chiefly to the left side of the nerve- 

 cord and connected posteriorly with a median siphon-shaped muscular tube. 

 The globular organ has walls partly muscular, partly glandular, and is called 

 consequently the prostatic part of the male intromittent apparatus. The 

 glands appear to secrete the material which forms the spermatophores. 

 The median siphon-shaped tube is the copulatory organ or penis, and its 

 walls contain both circular and longitudinal muscle fibres. From the base 

 of the prostatic body passes to right and left a ductus ejaculatorius. These 

 are each connected respectively to what is easily seen with attention to be 

 a mass of coiled tube of a yellowish colour. The coiled tubes, according 

 to Leuckart, contain at the height of the reproductive season numberless 

 minute globules. The masses in question may be regarded provisionally 

 as vesiculae seminales. Each coiled tube is continuous with a duct, the 

 vas deferens, which passes backwards parallel to the nerve-cord. It is 

 slightly tortuous. From its inner or median side nine branches arise, each 

 passing to a testis. The nine pairs of testes are globular bodies lying close 

 to the ventral nerve-cord, one behind each ganglion from the eighth to the 

 sixteenth inclusive. They are therefore segmentally arranged. The pro- 

 static apparatus, copulatory organ, and vesiculae lie in the tenth somite 

 (Whitman), the male aperture being median and ventral in the second 

 annulus of that somite. It is difficult to see unless the penis is protruded, 

 as it sometimes is in this Leech and in the common Horse-leech, Aulostoma 

 gulo, when the animal is killed by chloroform. The female organs lie in 

 the eleventh somite (Whitman), therefore in the somite interposed be- 

 tween the first pair of testes and the male intromittent apparatus. Close 

 behind the seventh ventral ganglion may be seen two roundish bodies, the 

 capsules which contain the true ovaries, lying one on either side of the 

 longitudinal nervous commissures. The oviduct is continuous with or 

 rather perforates these capsules. Its anterior part is forked, one branch of 

 the fork passing under the nerve-commissure to the left ovarian sac. The 

 posterior part is single and may be distinguished by its yellow colour. It 

 enters the base of an oval sac, the vagina, which has muscular walls and a 

 cuticular lining and opens by a median ventral aperture in the second 

 annulus of the eleventh somite. 



