niEUDIlTEA. 399 



draws its anterior end from this barrel-shaped membrane, which is 

 now filled and which, after the animal has left it, becomes in 

 consequence of the narrowing of the terminal openings a tolerably 

 completely closed cocoon. The number of eggs contained in a cocoon 

 varies but is never large. The eggs are small, yet the young 

 leeches when hatched are of considerable size, those of the Hirudo 

 medicinalis, for example, are about 17 mm. long, and, excepting 

 the fact that they are not sexually mature, have essentially the 

 organization of the adult animal. The young of Clepsine alone are 

 hatched at a very early stage, and differ essentially from the sexual 

 animal both as regards the shape of the body and the internal 

 organisation. They have a simple intestine, are without the 

 posterior sucker, and live a long time attached to the ventral 

 surface of the mother ; and it is not until they have received a 

 considerable quantity of newly secreted albuminous matter that 

 they obtain an organization which fits them to lead a free life. 



The development of the embyro of Clepsine among the Rhyncob- 

 dellidce and Neplidis and Hirudo amongst the Gnathobdellidce is better 

 known. The segmentation is always unequal. The mouth is formed 

 early, and through it, after the formation of the pharynx and intes- 

 tinal canal, the albumen contained in the cocoon is taken into the 

 intestine of the growing embryo by means of swallowing movements 

 of the pharynx. 



The Leeches live for the most part in water or temporarily in 

 damp earth. They move partly by " looping " with the help of 

 their suckers, and partly by swimming with active undulations of 

 the usually flattened body. Many of them are parasitic on the skin 

 or the gills of aquatic animals, e.g., on fishes and the cray-fish ; most 

 of them, however, are only occasional parasites on the outer skin of 

 warm-blooded animals. Certain forms are predaceous and, as for 

 example Aulastomum gulo, eat snails and earthworms, or, like Clepsine, 

 suck snails. They do not feed exclusively on any special genus of 

 animals, and their diet is not always the same in the different periods 

 of their existence. Hirudo medicinalis in its young stage lives on 

 the blood of insects, then on that of frogs, and only when it has 

 attained sexual maturity is a diet of warm blood necessary to it. 



Fam. Bhyncobdellidae. Leeches with proboscis. Body elongated, cylindrical, 

 or broad and flat ; with an anterior and posterior sucker, and a powerful pro- 

 trusible proboscis in the buccal cavity : with paired eyes on the anterior sucker. 

 Organs concerned in the formation of blood corpuscles occur (so-called valves) 

 in the dorsal contractile vessel. Piscicola Blainv. (Iclithyol)dellidfc}. P. cjcometra 

 L., on -fresh water fish. P. respirans Tr., with lateral vesicles which dilate as 



