I 



ECOLOGICAL 91 



all, or, on the other hand, rendering them any service. Among ecto- 

 parasites there are such forms as fish-Hce and many other Crus- 

 taceans, numerous insects such as lice and fleas, and Arachnids such 

 as mites. Among endoparasites there are Sporozoa, some Mesozoa, 

 many Nematodes, most Trematodes, all the Cestodes, many Crus- 

 taceans, insect larvae, and Arachnids. 



The parasitic habit implies degeneration (varying according to the 

 degree of dependence), great nutritive security, prolific reproduction, 

 and enormous hazards in the fulfilment of the life-history. 



Parasitic animals must be distinguished: (a) from epiphytic or 

 epizoic animals which live attached to plants or animals, but are 

 in no way dependent upon them, e.g. acorn-shells on Norway 



Fig. 25. 



Dimorphism in Bilharzia Worm (Schistosomum) . The male, about ^ inch 

 in length, carries, in a ventral groove, the longer, narrower female (F); 

 S, posterior sucker. After Looss. 



lobster; (b) from commensals, who live in some degree of external 

 partnership, but without in any way preying upon one another, 

 e.g. crab and sea-anemone; and (c) from symbions, who live in close 

 internal partnership, or symbiosis e.g. Radiolarians and Algae. But 

 between these habits there are many gradations, and from close 

 association there is always an easy transition to parasitism. 



TERRESTRIAL. — The colonising of dry land has doubtless been a 

 gradual process, as different types wandered inland from the shore, 

 or became able to survive the drying up of freshwater basins. The 

 fauna includes some Protozoa, e.g. Amoeba terricola, which lives in 

 moist earth, some of the Planarians, Nematodes, Leeches, Chaeto- 

 pods, and other "worms", a few Crustaceans like the wood-lice 

 (Oniscus), many insects and Arachnids, a legion of slugs and snails. 



