ENVIRONMENTAL FACTORS 297 



of such transfer normally perish as their habitats change. It is suffi- 

 cient merely to call attention to this matter here. 



Pedonic and limnetic organisms. — In the fauna of inland waters 

 as well as in that of the ocean one can distinguish between animals of 

 the bottom and those of the open water. It is natural for these groups 

 to retain the names benthic and pelagic, as they are designated in the 

 ocean; but it has become a common practice to speak of the bottom 

 organisms of the fresh water as pedonic and pelagic forms as limnetic. 

 Among the pedonic animals in fresh water there is also a distinction 

 between shore forms and animals found in greater depths; the deep- 

 water forms begin at the border of the plant growth. This boundary 

 varies with the transparency of the water and may begin at as low a 

 depth as 7 m. or may lie at a depth of 30 m. Since only a few inland 

 waters are so deep that it can be proved that no light penetrates to the 

 bottom, a really dark stratum with a truly abyssal fauna occurs in 

 only those few lakes whose depth is over 400 m., i.e., especially in Lake 

 Baikal and Lake Tanganyika. The giant planarians and the non- 

 pigmented fish (Comephorus baikalensis) of the former are truly 

 abyssal fresh-water animals. In the limnetic fauna of inland waters, we 

 again distinguish between the drifting plankton, and the nekton which 

 is independent and swims without the aid of water movements. The 

 nekton includes only vertebrates in fresh waters. 



The pedonic fauna of fresh water can hardly be distinguished by 

 fundamental differences from the corresponding fauna of the ocean 

 excepting perhaps in one characteristic, that sessile animals which are 

 so plentifully represented in the benthal of the ocean are almost en- 

 tirely absent, with the exception of several ciliates, the fresh-water 

 polyp Hydra, and a few attached insect larvae. The reason may be 

 that migration of sessile animals up river mouths against the current 

 is very difficult. The limnetic fauna, on the other hand, especially the 

 fresh-water plankton, has, in common with that of the ocean, impor- 

 tant adaptations which facilitate floating. Both the limnetic and the 

 pedonic environments are poorer in number of species than are corre- 

 sponding regions in the ocean, since so many types are completely 

 absent in fresh water, but although it is qualitatively poorer, quanti- 

 tatively it is much richer. The almost complete absence of larvae 

 and eggs is very noticeable in the composition of fresh-water plankton 

 in comparison with the plankton of the ocean. Only the mussel, Dreis- 

 sena polymorpha, which has been introduced into fresh water by 

 navigation, has planktonic larvae; besides these there are the flagellated 

 larvae of some Cestodes and the nauplii of many copepods. In open 

 water, the statoblasts of many Bryozoa and the winter eggs of many 



