236 PHYLUM ANNELIDA. 
by the mouth. Almost all are hermaphrodite,—the male 
organs are numerous and segmentally arranged, and special 
genital ducts are present. The genital openings are median. 
The development is direct. Most live in fresh water or on 
land, but a few are marine. 
Type, the Medicinal Leech (Airudo medicinalis) 
Habits.—This is the commonest and most familiar of 
leeches, once so constantly used in the practice of medicine 
that leech became synonymous with physician. It lives in 
ponds and sluggish streams, and though not common in 
Britain, is abundant on the Continent, where leech farms, 
formerly of importance, are still to be seen. Leeches feed 
on the blood of fishes, frogs, and the like, and are still 
caught in the old fashion on the bare legs of the callous 
collector. As animals are naturally averse to blood-letting 
and hard to catch, leeches make the most of their 
opportunities. They gorge themselves with blood, and 
digest it slowly for many months, it may be, indeed, for 
a year. Watched in a glass jar, the leech is seen to move 
by alternately fixing and loosening its oral and posterior 
suckers, and, on some slight provocation, it will swim 
about actively and gracefully. At times it casts off from 
its skin thin transparent shreds of cuticle,—a process 
which, in natural conditions, usually occurs after a heavy 
meal, when the animal, as if in indigestion, spasmodically 
‘contracts its body, or rubs itself on the stems of water- 
plants. Numerous eggs are laid together in cocoons in 
the damp earth near the edge of the pool. Thence, after 
a direct development, the young leeches emerge and make 
for the water. 
External features.—The leech is usually from 2 to 6 inches 
in length, amd appears cylindrical or strap-like, according to its state 
of contraction. The slimy body shows over one hundred skin-rings ; 
its dorsal surface is beautifully marked with longitudinal pigmented 
bands, while the ventral surface is mottled irregularly ; the suctorial 
mouth is readily distinguished from the unperforated hind sucker, above 
which, on the dorsal surface, the alimentary canal may be seen to end. 
According to Whitman’s precise investigations, there are 102 skin- 
rings and 26 somites or true segments. The hind sucker is supposed 
to consist of 7 fused segments, making the total number 33. 
