GENERAL NOTES ON LEECHES. 243 
GENERAL NoTEes 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 Oligochzetes. In leeches setz 
are absent, except in Acamthobdella, which has paired segmentally 
arranged bristles in the anterior region ; but it is to be noted that they 
are absent in some Oligochetes. As in Oligocheetes, gills are usually 
absent, but occur in Branchellion. The condition of the body cavity 
affords one of the most striking contrasts to Oligochetes; 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 Oligochzetes, except 
in a few forms, like Branchiobdella; the jawed leeches are more 
specialised than those without these structures, 
With regard to the nephridia, in Clepszze, 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 Azrudo, 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 Oligochzetes 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 (Arado) 
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 (Aacrobdella valdiviana), said to measure 24 ft. in length, 
though this is very doubtful, is subterranean and carnivorous ; while the 
wiry land-leeches (Hemadipsa, etc.), of Ceylon and other parts of the 
