GENERAL NOTES ON LEECHES. 215 



may remain for a long time within the uterus, or, liberated 

 from their packets, may work their way up the female duct 

 meeting the eggs at some point, or reaching them even in 

 the ovaries. 



The development is direct, and in many respects recalls 

 that of the earthworm. 



General Notes on Leeches. 



The leeches constitute a relatively small class, whose structure has been 

 insufficiently worked out. The presence of suckers, the parasitic habit, 

 the reduction of the body cavity, have led many naturalists to associate 

 them with Flat-worms, but all recent work goes to emphasise their 

 affinity with Annelids, especially Oligochcetes. In leeches sete are 

 absent, except in Acanthobdella, which has paired segmentally arranged 

 bristles in the anterior region ; but it is to be noted that they are absent 

 in some OligochEetes. As in Oligochaites, gills are usually absent, but 

 occur in Branchellion. The condition of the body cavity affords one of 

 the most striking contrasts to Oligochsetes ; but in Acanthobdella the 

 adult has a typical Annelid ccelom divided into regions by septa. In 

 others, in spite of the large amount of connective tissue in the adult, 

 there are distinct traces of segmental septa. In Hirudo the reduction is 

 carried so far that the ccelom is represented merely by canals without 

 trace of septa. In all cases, however, development shows that the 

 reduction is secondary, and that in the embryo there is a true Annelid 

 body cavity unconnected with the vascular system. The condition of 

 the alimentary canal affords a basis for classification, for in one set the 

 anterior region is protrusible, and in the other it is not, but is furnished 

 with jaws or tooth-plates. The jaws are interesting, because they are 

 absent from Oligochsetes, except in a few forms, like Branchiobdella ; 

 the jawed leeches are more specialised than those without these 

 structures. 



With regard to the nephridia, in Clepsine, which has a fairly well- 

 developed body cavity, there is a direct communication between ccelom 

 and nephridia by means of a ciliated funnel of typical Annelid form. 

 Where the ccelom is much reduced, as in Hirudo, the funnel is represented 

 by the blind ciliated "cauliflower lobe." In the reproductive system, 

 apart from the numerous male organs, the leeches differ from the 

 Oligochaetes in the apparent continuity of the organs and ducts ; in the 

 case of the ovaries at least, however, the connection is secondary. In the 

 processes of fertilisation and egg-laying, in the formation of a cocoon, 

 and in the development, the two groups show marked resemblance. 



Most leeches are worm-like aquatic animals, with blood-sucking 

 propensities ; but some live in moist soil, and others keep to the open 

 surface, while the parasitic " vampire " habit, familiarly illustrated by 

 the apothecary's ancient panacea, is in many cases replaced by 

 carnivorous habits and predatory life. The medicinal leech (Himdo) 

 is typical of the majority, for it lives in ponds and marshes, and sucks 

 the blood of snails, fishes, frogs, or of larger available victims. The 



