474 ZOOLOGY SECT. 



It will be noticed that the ovaries of the Leech form a single 

 pair, while the testes are multiple and segmental : also that, while 

 the gonads and efferent ducts of both sexes are paired, the penis 

 and the vagina are median and unpaired. In the latter respect 

 the contrast between the Leech and the Annulata previously dis- 

 cussed is very striking. Further important peculiarities are the 

 enclosure of the ovary in a sac from which a duct leads directly to 

 the exterior, and the fact that the testes are hollow sacs discharg- 

 ing the sperms into a cavity from which they pass directly to the 

 efferent ducts. In Chsetopods, it will be remembered, the gonods 

 lie freely in the ccelome, their products ova or sperms are dis- 

 charged from their external surfaces and carried off either by 

 ordinary nephridia or by nephridia specially modified into gono- 

 jjucts. It seems tolerably certain that the cavities both of the 

 Ovarian sacs and of the testes represent shut-off portions of an 

 almost obsolete ccelome, and that their ducts are meso-nephridia. 

 Development. When breeding two Leeches copulate^ and one 

 impregnates the other by passing spermatophore^ through its 



penis into the vagina, Simultaneous 

 mutual impregnation has also been 

 described. The clitellar segments 

 (ninth to eleventh) secrete a cocoon 

 (Fig. 378), into which spermatophores, 

 ova, and a quantity of albumen, se- 

 creted by the albumen-glands, are 



FIG. s-s.-The cocoon of mrudo. passed. The animal then withdraws 

 ^entire ; B, in section. (After ^s head from the cocoon, the two ends 



of which close up by their own elas- 

 ticity, producing a closed capsule in 



which embryonic development takes place. Segmentation is 

 unequal, and results in the formation of a globular embryo, which, 

 after hatching, swims about in the cocoon, actively devouring 

 its albuminous contents, and finally escaping in a form closely 

 resembling the adult. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Hirudinea are Annulata in which the body consists of a 

 limited and definite number of segments, and is marked externally 

 by secondary rings or annuli, a variable number of which go to 

 a segment. The anterior end of the body is suctorial, and several 

 of the hindmost segments are fused to form a powerful sucking 

 disc, which is directed downwards and backwards. The mouth 

 lies in the anterior sucker, the anus is usually dorsal and imme- 

 diately in front of the posterior sucker. The ccelome is always 

 more or less obliterated by connective-tissue, and is represented 

 by sinuses of varying dimensions which contain blood. True 



