38 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



land is frequent along the French Mediterranean coast and in the 

 northern Adriatic. Marine animals are caught in such basins and 

 thereby subjected to the influence of the inflowing fresh water. In so 

 far as they are stenohaline, they will be destroyed. Others, however, 

 will survive, and will then remain as forms with marine relations in 

 an association of fresh-water animals. These forms are marine relicts, 

 and such lakes are termed relict lakes. 



Lake Mogilnoje on the island of Kildin, on the Murman Coast, 

 Lapland, is a lake of this type in process of formation. 19 Although now 

 without visible oceanic connection, it is still so connected at greater 

 depths by seepage water. For this reason its waters are stratified, with 

 completely fresh water at the surface and to a depth of 6 meters; 

 from 6 to 12 meters there is a rapid increase in salinity; and below 

 this a uniform salt content of 32.5%o prevails. The lower stratum is 

 without animal life on account of the presence of hydrogen sulphide. 

 The upper stratum contains daphnias and fresh-water copepods, with 

 Gammarus locusta (an inhabitant of the marine beach) on the shore; 

 the middle layer contains only marine animals. In 1888, this marine 

 fauna consisted of two or three species of sponges, sea anemones, a 

 few bryozoa, snails and other mollusks, annelids, a sea star, four or 

 five tunicates, codfishes, and Pycnogonum. Ten years later a number 

 of the marine forms were no longer to be found alive, though their 

 remains were still present on the bottom. 20 



Marine relicts in such lakes often become very few in number. In 

 the lakes of southern Sweden, the only forms with marine relatives are 

 Mysis relicta and Limnocalanus ?nacrurus. In the lakes of the Baltic 

 divide, only Mysis relicta, Pontoporeia affinis, and Pallasiella quad- 

 rispinosa are reported. 21 American examples include Mysis relicta in 

 Lake Michigan and a nereid worm in Lake Merced near San Fran- 

 cisco. 



The special characters which enable animals to live in fresh water, 

 such as regulative activity of the excretory system, permeability of 

 the body membranes, and mucous covering of the skin, may be defined 

 as primary characters of fresh-water animals. A number of additional 

 characteristics of animals in fresh water have probably arisen through 

 the direct or indirect influence of this medium on their organization, 

 and may be defined as secondary characters. 



The first of these is the smaller size of fresh-water animals com- 

 pared with their marine allies. This applies to nearly all forms with 

 the exception of the rotifers. Thus the hydroid polyp Cordylophora 

 lacustris is smaller and has shorter stalks in fresh water than in brack- 

 ish. The fresh water Alosa of the Italian lakes is smaller than the 



