LEECHES 279 



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 OHgocha?ta. As in Oligocha^ta, gills are usually 

 absent, but occur in Branchellion. The condition of the body cavity 

 affords one of the most striking contrasts to Oligochaeta ; but in 

 Acanthobdella the adult has a typical Annelid coelom 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 coelom 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 Annehd 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 Oligochaeta, except 

 in a few forms, like Branchiohdella ; 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 coelom 

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

 Where the coelom is much reduced, as in Hirudo, the funnel is repre- 

 sented by the blind cihated " cauHflower lobe." In the reproductive 

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

 the 01igocha?ta 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 " vampure " 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 {Hcrniadipsa, etc.), of Ceylon and other parts of the 

 East, move very rapidly along the ground, fasten on to the legs of 

 man or beast, and gorge themselves with blood. The hungry horse- 

 leeches are species of Hcemopis, greedily suctorial, though the teeth, 

 which occur in two rows, are too small and irregular to be useful in 

 medicinal blood-letting ; but the name is also applied to species of the 

 common genus Aulostoma, which are carnivorous in habit. Other 

 common leeches are species of Nephelis, predaceous forms with indis- 

 criminating appetites, and the little Clepsine, also common in our 

 ponds, notable for carrying its young about on its ventral surface. 

 Several marine forms prey upon fishes and other animals, e.g. the 

 " skate-sucker " {Pontobdella muricaia), with a leathery skin rough with 

 knobs. This form lays velvety eggs in empty mollusc shells, and 



