Medicinal Leech. 135 



organs arranged opposite the interspaces ol' the testes, and two in 

 the two anterior genital segments. Four more are to be seen in 

 the segments anterior to those last named, and the three first of 

 these are in closer proximity than the rest of the series. Tliei-e 

 are three segmental organs in the segments posterior to tliat 

 containing the last testis ; and they possess a coecal jorocess pro- 

 longed inwards beyond the line of the globular sac, by which their 

 excretion is discharged on the exterior of the body. The coecal 

 process of the most anterior of the three, comes into apposition 

 with the posterior testis, but its homologue is not developed in 

 the six anterior segmental organs. The segmental organs are 

 much larger in the Medicinal Leech than in tlie Horse-leech ; and 

 Gratiolet connects the greater power which the former animals 

 have of living out of the water with the greater power of moist- 

 ening the skin thus attained. Hirudineae, such as Branchellion, 

 which possess only two pairs of segmental organs, and Nephelis 

 and Clepsine, in which these organs attain but a small develop- 

 ment, appear never to leave the water, as the other genera do, 

 spontaneously ; and in them, it should be added, the segmental 

 organs open, which they do not in either of the Leeches men- 

 tioned, by ciliated infundibula into the general cavity of the l:>ody. 

 The various portions of the loop-shaped constituent of the seg- 

 mental organs of the Leech, communicate with each other very 

 freely by lateral branches of anastomosis, which make the gland 

 to be labyrinthiform rather than merely tubular. It is of im- 

 portance both to the morphology and to the physiology of these 

 animals, to observe that the failure of the organs of vegetative 

 liCe here described to be developed at either end of the body, 

 coincides with an aggregation both of segments and of animal 

 organs in the same two regions. The azygos character of the 

 generative ducts is noteworthy, as is also the development of an 

 intromittent organ ; a structure not found in other Annelids, 

 though existing both in Platyclminthes and Nematelminthes. 



For the segmental organs, see (iratiolct, Ann. Sci. Nat. iv. 17, 

 p. 192, 1>1. vii., fig. 4; Leuckart, Die Menschliclien Parasiten, i., 

 p. 672. 



For the reproductive organs, see Leuckart, I. c, p. 673; and for 

 the antagonism which subsists between the evolution of these 



