Zonation and Stratification 55 



Prenant 6C Tessier (1924) have made a very careful study of the 

 zonation of the sessile animals on the beaches at Roscoff, France, 

 and cite the following factors which appear to be most important 

 in limiting distribution: (1) Water level; a few species stand desic- 

 cation, but most are continually submerged. (2) Light; some species 

 avoid light, apparently because their larvae are usually negatively 

 phototropic. (3) Freshwater; hydroids and bryozoans have little 

 resistance to dilution; some barnacles have much. (4) Sediment; 

 some species depend on this and organic food in the water. (5) 

 Clearness; some species are limited to clear water. (6) Nature of 

 substratum; a very important influence, as Allee (1923b) also 

 found. (7) Depth influences, desiccation, wave action, sediment, 

 light, and other factors as well as pressure. (8) Danger, which 

 these authors think is most important of all; most species they find 

 to be arranged according to this. On the coast of Canada, Fiunts- 

 man (1918a) found that vertical distribution of several littoral 

 animals was limited largely by enemies. 



Animals along the shores of all sorts of aquatic environments 

 continually tend to spread into terrestrial habitats but are usually 

 prevented from doing so by various factors. For example, Lieber 

 (1931) thinks that limnodrilids might gradually become land ani- 

 mals if it were not for occasional high waters and frosts which pre- 

 vent them from doing so. The evidence that many animals have 

 left the sea and attained land life through the intertidal zone has 

 already been reviewed. In this section the view is stressed that 

 segregation into habitats, particularly zonation and stratification 

 along beaches, has been a factor in the development of terrestrial 

 animals. 



Food 



All animals require organic food. This they secure directly or 

 indirectly from plants which are able to use radiant energy to manu- 

 facture new molecules. A few plants are able to build organic 



