x PHYLUM ANNULATA 503 



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 

 discussed 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 discharging 

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

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

 lie freely in the ccelome, and 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 

 ccelome, 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 

 also been described. The clitellar seg- 

 ments (ninth ^to eleventh) secrete a 

 cocoon (Fig. 419), into which spermato- 

 phores, ova, and a quantity of albumen, 

 secreted by the gland-cells in the wall 

 of the median oviduct, are passed. 

 The animal then withdraws its head 



, , FIG. 419. Thecocoon'of Hirudo. 

 from the COCOOn, the two ends Of A, entire ; B, in section. (After 



which close up by their own elasticity, 



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 CHARACTEES 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 coelome is always 

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

 by sinuses of varying dimensions which contain blood. True 

 blood-vessels, with muscular walls, are also present. The nervous 

 system consists of a brain united by short ossophageal connectives 

 to a ganglionated ventral nerve-cord. The excretory organs are 



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