VERMES 145 



suckers, a large one at the posterior end of the body, and a 

 smaller one at the anterior end surrounding the mouth. Near 

 the posterior end of the body on the dorsal side is a small open 

 ing, the anus, near the posterior sucker. On the dorsal side <>| 

 the anterior portion of the body are live pairs of small eyes, and 

 in one marine genus there is a row of gills or branchiae al< 

 each side of the body. Some leeches are provided with a sort 

 of proboscis corresponding to the introvert of the Gephyrae, 

 while others have instead three jaws in the mouth, a median and 

 two lateral, which are moved by strong muscles and provided 

 with numerous small, chitinous teeth. 



The internal structure differs in many respects from that of 

 the typical Annelida (Fig. 137). The alimentary canal is pro- 

 vided with a large crop or food reservoir, 

 with symmetrically arranged pouches, 

 which is capable of great distention ; and 

 when this is filled with blood from its 

 victim, the process of digestion goes on a %rs B 



very slowly, a year being sometimes con- F]G I38 Hirudo, cocoon, 

 sumed in this function. There are a, surface view ; b, in section. 



(After Leuckart, from Parker 



symmetrically arranged nephndia, germ- and Hasweii's Text-book.) 

 glands, lateral blood vessels, and nerve 



ganglia, the nervous system having the same general structure 

 as in typical Annelida. In the Hirudinea the number of seg- 

 ments in the body is constant, there being thirty-four. One great 

 difference between this class and the other jointed worms is found 

 in the fact that the body-cavity is almost entirely obliterated, so 

 that in most cases it can be recognized merely as rather incon- 

 spicuous spaces forming a part of the blood system. The leeches 

 are hermaphroditic ; there are several pairs of testes whose ducts 

 ultimately connect with the single copulatory organ, or penis, on 

 the median ventral line of the body; there is only a single pair 

 of ovaries. Cross fertilization takes place, and the eggs are 

 deposited in a porous cocoon (Fig. 138) secreted by a sort of 

 clitellum like that of the earthworms. 



