200 ASSOCIATION OF ORGANISMS—THE WEB OF LIFE 
Crayfish (Astacus fluviatilis), for example, being infested by 
several of them (species of Branchiobdella), which suck its blood 
and devour its eggs. 
Leecues (Disco- 
PHORA) AS PARASITES. 
—While some leeches 
prey upon animals 
smaller than them- 
selves, others are true 
external _ parasites, 
and the habits, of 
these have been suf- 
ficiently described 
elsewhere (see vol. ii, 
p. 147). Some of 
these creatures (e.g. Prsczcola, fig. 1148) attach themselves to 
the exterior of fishes, their presence causing great annoyance or 
even proving fatal. 
Fig. 1148.—Fish-Leeches (Pzscico/a) attached to the Head of a Carp 
FLUKES (TREMaATODA) AS PARASITES 
The flattened unsegmented animals included in this group are, 
almost without exception, of parasitic habit. They are provided 
with organs of adhesion in the form of suckers, and the mouth 
leads into a muscular pharynx, which serves as a sort of suction- 
pump by which blood and other substances are taken into the 
body. 
As ectoparasites, flukes are only found upon the bodies of 
aquatic animals, and in this case three or more suckers are pre- 
sent, since efficient means of holding on are clearly a matter of 
primary importance. The gills of fishes are particularly liable 
to such attacks, and it is only natural that this should be so, for 
their sheltered position, delicate texture, and abundant blood- 
supply are great advantages, from the parasitic point of view. 
We may take as an example a form (Octobothrium pollachit, 
fig. 1149) which lives upon the gills of the pollack, adhering by 
means of eight stalked suckers. A related species (O. merlangt) 
lives on the whiting, and the herring is infested by a similar 
parasite, in which, however, the suckers are not stalked. A 
curious case, where the host is not a fish, is presented by a 
