GENERAL NOTES ON LEECHES. 243 



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 Oligochaetes. In leeches setre 

 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 Oligochaetes. As in Oligochaetes, 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 Oligochaetes, 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 repre- 

 sented 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 ; 

 but in the case of the ovaries, at least, 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 (Hirudo} 

 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 

 giant leech (Macrobdella valdiviana\ sometimes measuring i^ ft. when 

 at full length in movement, is subterranean and carnivorous ; while the 

 wiry land-leeches (Hcemadipsa, etc.), of Ceylon and other parts of the 



