ENVIRONMENTAL FACTORS 



299 



is, however, impossible without damage to the organism (cf. p. 34 f.). 

 We know of gelatine formation in only a few cases in fresh-water ani- 

 mals, and in these only lifeless parts of the body are expanded, e.g., 

 the mantle of the water flea Holopedium (Fig. 79) and a few rotifers 

 (Fig. 80) . The increase of surface area by means of thread-like pseudo- 

 podia, such as serve as aids to floating in Radiolaria and Foraminifera 

 in the ocean, though it is found as in the Heliozoa, occurs much more 

 rarely among fresh-water animals, perhaps because with such greatly 

 increased surface area the amount of fresh water absorbed would be 

 too great. Non-gelatinous plankton animals also reach a greater size 

 in the ocean than in fresh water. 

 The reason lies in the lesser density 

 and the consequent lessened buoy- 

 ancy of fresh water as compared 

 with ocean water. 23 The skeletons 

 of fresh-water plankton animals 

 rarely contain lime. The largest 

 animal of the fresh-water plank- 

 ton, the larva of the Corethra 

 plumicornis (Fig. 102), which 

 reaches a length of 15 mm., has 

 two pairs of air-filled tracheal 

 bladders which enable it to float. 

 The minimum size, however, is 

 about the same for the fresh-water 

 plankton as for that of the ocean; 

 there is a dwarf plankton in both 

 regions. 



The secondary inhabitants of 

 this medium, those which have re- 

 turned from the terrestrial to the aquatic habitat, are more plentiful 

 in fresh water than in the ocean. They include snails, insects, arachnids, 

 and vertebrates. The varied conditions of water movement, and the 

 necessity for getting oxygen, have brought about many convergent 

 structures, e.g., the flat, sharp-edged body shape of water beetles 

 (Dytiscidae and Hydrophilidae) and water bugs (Naucoridae) ; the 

 swimming legs, widened by means of hairs; the lightening of the body 

 by means of carrying air on the hairy abdominal surfaces, among 

 beetles (Hydrophilus) and bugs (Notonecta) ; the development of long 

 breathing tubes which reach to the surface, among animals of shallow 

 waters, e.g., some water bugs (Nepa, Ranatra) and the rat-tailed mag- 

 got of the hover fly (Eristalis) . Many vertebrates which mainly seek 



Fig. 81. — Heads of a frog, a croc- 

 odile, and a hippopotamus as ex- 

 amples of convergent adaptation to 

 amphibian life. 



