160 



Gust. Alsterberg 



vertically, with the strongest saprobiotic phases at the bottom and the least saprobiotic 



at the surface. 



E. They are eurytrophic, hut show increase of numbers in the eutrophic zone. 



F. They are to be ranged under the Sapropelic fauna and, with the Chironomid- 

 larvse and other animals, form the episapropelic (Surface-sapropelic) type of this fauna. 



II. Habitat. 



A. The habitat of the Tubificidœ comprises two media — mud and water. 



B. The mud constitutes the nutritive medium of the Tubificidœ, but is un- 

 serviceable as a respiratory medium, for, according to what experiments have shown, 

 it is completely destitute of 0,, a fact that is due to the following conditions: — 



1) That even under the most favourable conditions, the superincumbent water 

 contains little 0 2 . 



2) That the dissolved 0 2 cannot be transported down into the mud except 

 by diffusion. 



3) That the diffusion of 0 2 proceeds very slowly. 



4) That, finally, the small quantity of 0 2 that is able to diffuse into the mud 

 is instantly consumed by the micro- organisms, or is reduced by oxidable substances 

 present there. 



C. The water above the mud must constitute the respiratory medium of the 

 Tubificidœ, this medium being exceedingly unstable in respect of 0 2 on account 

 of the saprobiotic processes emanating from the mud. 



1) The saprobiotic processes may be divided into: — 



a) The previously mentioned diffusion of 0 2 directed towards the mud. 



b) The exhalations of oxidable substances, which emerge from the mud, where 

 they are produced. 



c) The emigration of organisms, which leave the mud on experiencing lack of 0 2 . 



2) The saprobiotic processes manifest themselves: — 



a) Locally, by the formation of layers, it then being possible for the cecological 

 spectra to be projected vertically. The two forms of local instability or layer- 

 formation are : — 



a) A respiratory macro-layerformation, which is characterized by the respira- 

 torily heterogeneous layers being located far apart. 



ß) A respiratory micro-layerformation, which constitutes the preliminary stage 

 of the former extreme type and is the most generally realized form of local insta- 

 bility. In this there is, at minimum differences of level above the mud -line, great 

 divergence in the concentrations of 0 2 . 



b) Temporarily, in successive phases, which, with respect to local instability, 

 may be seen from two points of view : — 



a) Each layer may be converted from the one type of layer to the other — 

 micro to macro and vice versa. 



