200 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



Crayfish (Astac^ts ftuviatilis), for example, being infested by 

 several of them (species of Branchiobdella), which suck its blood 



and devour its eggs. 

 LEECHES (Disco- 



^L PHORA) AS PARASITES. 



M ?. /ilk -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. Piscicola, fig. 1148) attach themselves to 

 the exterior of fishes, their presence causing great annoyance or 

 even proving fatal. 



Fig. 1148. Fish-Leeches (Piscicola] attached to the Head of a Carp 



FLUKES (TREMATODA) 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 (Octobotkrium pollachii, 

 fig. 1149) which lives upon the gills of the pollack, adhering by 

 means of eight stalked suckers. A related species (O. merlangi] 

 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 



