MEDICINAL LEECH. 319 



PLATE XIII. 



FIGURE OF MEDICINAL LEECH (Hirudo medicinalis), 



Dissected so as to show its nervous, digestive, reproductive, and .segmental organs, as seen from 

 below; slightly altered from Moquin Tandon's figure, PL VIII., Fig. 10, Monographic des 

 Hirudinees, 1846. 



THE integument is drawn as divided down the middle ventral line, 

 from the posterior border of the buccal cavity or anterior sucker to the 

 anterior border of the posterior sucker ; two of the testes and two of the 

 nephridia have been displaced outwards in the somite, lettered d 3, e 2 ; 

 the rest of the organs have been left undisturbed in situ. 



a i. Anterior sucker. 



a 2. Posterior sucker, formed by the fusion of seven distinct somites, to 

 which as many ganglia, subsequently fused into the single posterior 

 ganglion of the ventral chain, corresponded at one period of the 

 animal's development. The anus is dorsal and anterior to the 

 sucker. 



b i. Infra-oesophageal ganglion and first ganglion of ventral chain, very 

 closely apposed to each other. 



b 2. Last ganglion, the twenty-third of the ventral chain, composed of 

 seven embryonic ganglia fused. This ganglion gives off from five 

 to nine pairs of nerves, which are distributed to the posterior sucker. 

 The penultimate ganglion gives off only one pair of nerves. 



c i. First lateral diverticulum of the portion of the digestive tract, which 

 comes next after the pharynx. 



c 2, ' Small intestine ' of most authors, ' gastroil^al ' portion of digestive 

 tube of Gratiolet, in which the haemoglobin of the blood undergoes 

 changes. It ends posteriorly in a short ovoidal colon, which again 

 ends in a short rectum, turned slightly upwards to the anus. The 

 small intestine is a little dilated at its commencement in the interval 

 between the two terminal sacculi c 3. This dilatation is much larger 

 and more distinctly bilobed in the Horse-leech (Aulostoma Giilo\ 



c 3. Eleventh lateral diverticulum of right side prolonged downwards on 

 either side of the small intestine and colon, as far as the point where 

 the rectum begins. 



d i. The most anteriorly placed of the nine testes of either side, commu- 

 nicating by a short transverse duct passing outwards, with a vas defe- 

 rens common to it and the eight posterior testes, and anteriorly con- 

 voluted. These convolutions are seen in this figure in the space on 

 the right side bounded by the lines lettered / and/! 



