"62 General Part. 



ileteropoda, Salpidse) . In other cases^ the surface is increased^ either 

 by the flattening of the body, by the lengthening of the limbsj or by 

 the formation of long spines, as in many Crustacea and their larvae, 

 and in young Fish : in both cases it is found that the muscles causing 

 the movement of the animal are reduced. Sometimes the atrophy 

 goes so far that they become exceedingly weak, and the animal 

 is only capable of feeble movements. In other cases, in spite 

 of the weakened muscles, they are still good or even excellent 

 swimmers, since the body floats in the water almost without muscular 

 exertion. The majority of pelagic forms are further characterised 

 by a great transparency. Many possess as perfect optic 

 organs as most of their relatives, as in the case of certain pelagic 

 Chsetopods : others, on the contrary, exhibit degenerate eyes, which 

 is explained by the fact that many members of the pelagic fauna 

 appear on the surface of the sea only at night, remaining in greater 

 •depths by day. 



Many animals remain for their whole life near the place where 

 they were born: others stray about in search of food: others, again, 

 undertake further migrations, usually in great numbers. 



These wanderings have often a casual and irregular character. 

 Unfavourable conditions, for example, scarcity of food or of water, 

 will cause an animal to leave the usual habitat of the species, and 

 to seek a new home. The migrations of Locusts, of Prairie-hens, of 

 the Lemming, result from such causes. These wanderings but seldom 

 increase the range of a species : after a short time it disappears 

 again from the new place. 



The migrations may, however, have a more regular character, 

 such as those undertaken annually by Pish and Birds: at one 

 time of year they inhabit one place, at another time another place, 

 from which they then return to the first, and so on. Among many 

 Pish, the migrations are due to a search, at a fixed time of year, for a 

 suitable breeding- ground; whilst the flight of Northern Birds towards 

 the South is really explained by -their not finding enough food for 

 the winter in their summer home. (For details see these groups.) 



2. Different kinds of Pood and their effect on 

 the form of the Body.— Parasitism. 



The food of animals is of very different kinds. It may be 

 vegetable, consisting of living or dead plants; or animal, 

 consisting of living animals or carrion. Some animals eat a 

 variety of things both animal and vegetable, others are, however, 

 much restricted, e.g., to a few species of plants. Some feed on 

 organisms of relatively very small size, others consume animals 

 larger than themselves. 



