x PHYLUM ANNULATA 515 



which are supplied with albumen-secreting gland-cells. The 

 common oviduct opens into a curved muscular tube, the vagina (va.), 

 which opens in the middle line on the ventral surface of the 

 fourth annulus of the twelfth segment, i.e. one segment behind 

 the male aperture. 



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 Ctuetopods, it will be remembered, the gonads 

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

 charged from their external surfaces and carried off either by 

 coelomoducts or by " segmental organs." It seems tolerably 

 certain that in the Leech the cavities both of the ovarian sacs 

 and of the testes represent shut-off portions of an almost 

 obsolete coelome, and that their ducts are coelomoducts. 



Development.- -When breeding two Leeches copulate, and one 

 impregnates the other by passing spermatophores through its 

 penis into the vagina. Simultaneous mutual impregnation 

 has also been described. The clitellar segments (ninth to 

 eleventh) secrete a cocoon (Fig. 412), 

 into which spermatophores, ova, and a 

 quantity of albumen, secreted by the 

 albumen-glands, are passed. The animal 

 then withdraws its head from the cocoon, 

 the two ends of which close up by their 

 own elasticity, producing a closed cap- 

 sule in which embryonic development FIG. 412. The cocoon of Hirudo. 



11 o j_ i A, entire : B, in section. (After 



takes place. Segmentation is unequal, Leuckart.) 



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 



L L 2 



