NATURAL HISTORY OF BRITISH FRESH-WATER LEECHES. 67
containing leeches, in such a manner as to reverse the direction
of the impulses. The leeches swam away from him.
Each species seems to have its favourite diet, and in some
cases the variety of food is very limited. Piscicola geometra
and Hemiclepsis marginata are parasitic, as a rule, upon fresh-
water fish. It has recently been demonstrated by Miss M.
Robertson (38) that certain trypanosomes found in the blood of
perch, bream and goldfish complete their life-cycle in the latter
species of leech. Protoclepsis tessellata, the Chequered-leech,
feeds upon the blood of various species of ducks. The favourite
food of the two British species of Glossosiphonia is fresh-water
Gastropods. The members of the group, Arhynchobdellae, with
the exception of the Medicinal Leech, prey upon worms, aquatic
larvae, frogs and sickly fish, indulging in a comparatively
varied diet. The Medicinal Leech is the only British species
which is able to pierce the skin of mammals. This specialisa-
tion in diet is correlated with peculiar modifications in the
physiological processes of digestion and in the structure of the
alimentary canal. Haycraft (24) has shown that a fluid is
secreted in the mouth which prevents the coagulation of the
blood. It was found that the action was the same in the case
of the blood of the rabbit and dog as in that of man. The
secretion is, however, unable to prevent the coagulation of
crustacean blood. The alimentary canal of the Medicinal Leech
is provided with lateral pouches which serve for the storage of
food. Spiess (41) has shown that specialisation extends
further than this. Special muscles are developed in the pharynx
which enable suction to be performed. The ordinary digestive
ferments are not secreted and the intestinal epithelium is the
same in character throughout almost its entire length. Com-
paring this with the digestive tract of its near ally, the Horse-
leech (Haemopis sanguisuga), whose diet consists partly of solid
food in the form of earthworms and similar creatures, and partly
of the blood of frogs and large worms, we find a different state of
affairs. No anti-clotting secretion is present, the alimentary
canal is not provided with lateral pouches, and special muscles
for the purpose of suction are not present. Moreover, the
intestinal epithelium secretes digestive ferments and the epi-
thelium is differentiated in the various regions.
Leeches show some very remarkable features in their modes