800 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 
(see Beddard’s Azdmal Coloration). Our only answer at present is 
that there is need for experiment. 
(c) Parasitic fauna.—It seems legitimate to rank together those 
animals whose habitat is in or on other organisms, from which they 
derive subsistence, without in most cases killing them quickly, if at all, 
or, on the other hand, rendering them any service. Among ectopara- 
sites there are such forms as fish-lice and many other Crustaceans, 
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 Crustaceans, 
insect larvee, 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 lobster ; (4) from 
commensals (p. 178), who live in some degree of partnership, but without 
in any way preying upon one another, ¢.g. crab and sea-anemone ; and 
(c) from symbions, who live in close partnership, or symbiosis (p. 119), 
e.g. Radiolarians and Algee. 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 fresh-water basins. The fauna includes some Pro- 
tozoa, eg. Ameba terricola, which lives in moist earth, 
some of the Planarians, Nematodes, Leeches, Cheetopods, 
and other “ worms,” a few Crustaceans like the wood-lice 
(Onzscus), many insects and Arachnids, a legion of slugs 
and snails, most adult Amphibians, most Reptiles, many 
Birds, and most Mammals. Among Vertebrates certain 
fishes are of interest in having learned to gulp mouthfuls 
of air at the surface of the water, to clamber on the roots 
of the mangrove trees, or to lie dormant through seasons of 
drought. But among Vertebrates, Amphibians were the 
first successfully to make the transition from water to dry 
land. 
It is important to bear in mind that many a stock may, in the course 
of its evolution, have passed through + variety of environments. Thus 
the thoroughly aquatic Celaceans were probably derived from a land 
stock common to them and to the Ungulates, and may have passed 
through a fresh-water stage. Without going farther back, we have 
here an illustration of the zigzag course of evolution. 
