88 LIFE : OUTLINES OF GENERAL BIOLOGY 



and minnows, water-snails and freshwater mussels, an occasional 

 water-spider and many water-mites, small crustaceans, leeches, 

 wheel animalcules, Turlx'llarian worms, Hydra, Spongilla, Infusori- 

 ans and Amcebce — a ver\' representative series of animals. Some of 

 them, such as shalU)W-water molluscs, have free-swimming larvae in 

 the open lake. 



The oyicn water includes a multitude of minute Algae which form 

 the chief f(H)d-supply of the small drifting or Plankton animals, 

 notably the Kntomostracan Crustaceans, popularly called water-fleas. 

 Far from shore there may also be great growths of the rootless 

 Utricularia, jxirtly insectivorous, and sheets of the Water Lobelia. 

 Also able to effect photosynthesis are a number of greenish Protozoa, 

 e.g. Peridinium and Ceratium, which have chlorophyll, or sometimes 

 partner Alg.e. Of great interest is the waxing and waning of the lake- 

 plankton, varying .sometimes with temperature and weather, some- 

 times with changes more obscure — even with the moon. To be 

 distinguished from the drifting Plankton is the more energetic, 

 relatively more katabolic Nekton, such as very active water-mites 

 at one extreme and large lake-trout at the other. In Lake Baikal 

 there arc actually .seals, pointing to an ancient connection with 

 the sea. 



Thirdly, the dark deep waters of the lake contain characteristic 

 animals, such as the Rhizopod Arcella, a rcddi.sh species of Hydra, 

 a considerable variety of Planarian, Nematode, and Oligochaete 

 worms, a blind C\clops, a deep-water bivalve (Pisidium) and snail 

 (Limna'U-^), some light-shy mites, and a few fishes like the sluggish 

 Burbot {Lota vu/f^aris) and the giant barbel (Silurus). 



Intkr-Ki:l.\tions. — The economy of a lake or a pond depends 

 on the balance between the photosynthetic "producers" and the 

 animal "consumers", witli bacteria as links that convert the remains 

 or waste-{)roducts of animals into nitrogenous substances and carbon 

 dioxide which may be utilised by green plants. The fact that some 

 animals devour others and may be themselves devoured in turn, 

 does not affect the general proposition that the animal population 

 deix-nds in the long run on tlie plants. As in the .sea, the "nutritive 

 chains  may be long or short: thus mergan.ser — trout — water-snail 

 — water-wred would be an average chain of four links, while 

 swan - water-weed would Ix? a common short one. 



Many of the inter-relations are of great interest. Thus the fresh- 

 water mussels rerpiire for the continuance of their race the uncon- 

 scif)us co-oixration of some freshwater fish, such as the minnow, on 

 the .skin or gills of which the ( "glocliidia") larvse of the mollu.sc fix 

 themselves and pass through a metamorphosis, eventually dropping 

 off into the mud, jx^rhaps far from the place where they effected 

 fixation. On the other hand, there can be no continuance of the 

 Bitterling [Hlwdcus a warns) unless the female fish, which has a very 



