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 
larvas, 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 ( Hczmopis 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 
