COMMUNITIES IN STANDING WATERS 355 



in water but care for them until they are hatched. 95 The frogs 

 of the genera Dendrobates and Phyllobates in tropical America pro- 

 vide for the drying-up of their habitat in a unique way; at the be- 

 ginning of drought the tadpoles, 6-7 mm. in length, attach themselves 

 by means of suckers to the backs of their parents and are thus carried 

 to another body of water. 90 



The moss fauna. — Convergent adaptations to frequent drying-up 

 of a habitat niche appear most often in the animal communities of 

 the moss on rocks, trees, walls, and roofs. 92 These moss beds may 

 be considered here as particular examples of small periodic bodies of 

 water. While the change from damp to dry is usually very frequent 

 in them, the changes in temperature also are especially great. The 

 opportunity for the development of active life, which is brought about 

 in moss by a rain, is so brief at certain periods of the year that it 

 often is too short for the animals to develop from eggs to egg-laying 

 or sexually mature adults. Therefore, not only the eggs but also the 

 adults of the inhabitants of moss beds must be insured against death 

 by desiccation. Thus all the animals of the temporarily damp moss 

 beds survive drought anabiotically. They can dry up to a piece of dust 

 without sacrificing their power to revive. A periodic sexual cycle is 

 necessarily absent. All activity depends directly on wetting and drying 

 of the moss environment. 



The inhabitants of these periodic fresh-water societies belong to 

 the rhizopods, the threadworms, the rotifers (especially Bdelloidea of 

 the genera Callidina and Philodina) , bear animalcules (Tardigrada), 

 and copepods (Harpacticidae) . Associated with these are terrestrial 

 animals which are not affected by the drying-up of the habitat, e.g., 

 minute insects and mites. Most of these, with the exception of bear 

 animalcules which suck the juices of moss plants, are detritus feeders. 

 Nematodes and bear animalcules may resort to the predatory habit. 

 Some means of attachment is usually present as an adaptation to 

 this type of habitat. This keeps the tiny animals or their eggs from 

 being washed out of the moss during heavy rains. The Tardigrada 

 have strong claws on their feet, and occasionally long, bristle-like 

 threads on their upper surface (Fig. 104) ; the rotifers have cement 

 glands in their feet, secreting a substance by which they attach them- 

 selves. The eggs of many species of the former (Fig. 105) are entirely 

 covered with stickers and spines and are thus anchored in the moss; 

 other species lay smooth eggs into their cast skins, which are en- 

 tangled in the moss by means of the claws. 



The variation in temperature to which moss-dwelling animals are 

 exposed is exceedingly great in many places, since the moss is often 



