ECOLOGICAL 143 



found on a common British bivalve (Cyprina islandica) does not 

 seem to do more than capture a share of inswept organisms. 



In the phylum or series of round-worms (Nematohelminthes) 

 numerous Nematodes are truly parasitic, while the others are sapro- 

 phytic ; but it is difficult to draw the line, for many of those inhabiting 

 the alimentary canal of higher animals find their whole nourishment 

 in the half-digested food or in the putrefying undigested residue. 

 Many of these internal saprophytes, for that is what they should be 

 called, are active and even agile. It is otherwise with the allied and 

 formidable hookworm, which sucks blood from the wall of the 

 food-canal, and also with the gapes-worms, which grip and suck, 

 even to choking, the windpipe of chickens and young pheasants. 



Fig. 38. 



Sex Dimorphism in the Green Marine Worm Bonellia. The female (F) has a 

 body about the size of a prune, while the pigmy male (M) is of microscopic 

 dimensions, about 1-5 mm. in length. 



Distantly allied to Nematodes are the Echinorhynchus types, 

 thoroughgoing parasites without mouth or food canal. They are 

 digenic, i.e. requiring to pass from one host to another if their life- 

 cycle is to be completed. In this case the passage is from Arthropod 

 to Vertebrate ; thus Echinorhynchus gigas of the pig passes its larval 

 stage in the grubs of cockchafers and the like; and E. proteus of 

 pike, trout, and minnow has its larva in the common freshwater 

 crustacean (Gammarus pulex). 



Parasitism is very rare among Ch?etopod worms, but the mouth- 

 less pigmy males of Bonellia and its ally Hamingia live — surely as 

 parasites — within the oviduct of the females. Other curious forms 

 (Discodrilida) are found on freshwater crustaceans, Branchiobdella 

 on the gills of the common European crayfish {Astacus fluviatilis) 

 and its American ally on the corresponding "crayfish" (Cambarus) 



