Ecological animal geography; an authorized, Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 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, , 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
Ecological animal geography; an authorized, Ecological animal geography; an authorized, rewritten edition based on Tiergeographie auf ockologischer grundlage ecologicalanimal00hess Year: 1937 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, , 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 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, , 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 th
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