40 ECOLOGICAL FOUNDATIONS OF ZOOGEOGRAPHY 



of the west coast of France would die out if they were not constantly- 

 replenished with spawn from salt water. The brackish-water polyp, 

 Cordylophora lacustris, in fresh water suffers a decrease in the num- 

 ber of gonophores and an approximate halving of the number of eggs 

 in each gonophore. 26 



The fact that egg size increases in fresh water is especially evident 

 in species which also occur in brackish water and in the sea. The 

 small crustacean, Palaemonetes varians, in specimens of equal size 

 (4 cm. length), has 321 eggs, of little over 0.5-mm. diameter, in salt 

 water, and only 25 eggs of 1.5-mm. diameter in fresh water. The egg 

 size is in the proportion of 1:27, and in total mass the fresh-water 

 form has produced twice as much of egg material as the marine, so 

 that the small number of eggs cannot in this case be a result of general 

 degeneration. In most fishes common to the North Sea and the Baltic, 

 the egg size increases with reduction of salinity of the water; for 

 example, in the flounder, Pleuronectes platessa: 



Salt content 19.45 17.3 15.68 % 



Egg diameter 1 .876 1 .901 1 .953 mm. 



In the Baltic starfish, Motella cimbria, the slight increase in size in 

 the fresher water suggests a mere swelling of the eggs in the weaker 

 salt solution. 



The abundance of yolk in fresher waters has the result that the 

 young animals hatch at a more advanced stage, and a free-swimming 

 larval stage is thus suppressed to a greater or lesser degree. In Palae- 

 monetes varians, the young zoea larvae are 4 mm. long and slender at 

 hatching in the marine forms, and 5^2 mm. long and stout in the fresh- 

 water form. 27 The herring (in Schleswig) hatches in a noticeably more 

 advanced state in brackish than in salt water. 28 Free-swimming larvae, 

 among fresh-water forms, are present especially in the copepods, the 

 polyphemid Cladocera, and the recent arrival from salt water, the 

 triangle mussel (Dreissena poly?nor-pha) . 



Another explanation for the absence of larvae in so many fresh- 

 water animals has been advanced by Sollas. He believes that free- 

 swimming larvae, which are capable of only weak active movements, 

 would always be carried to the ocean by the current of streams which 

 they tried to enter, and that therefore only animals without such 

 larvae, i.e., with large-yolked eggs and abbreviated development, 

 would be able to enter rivers from the sea. These two explanations are 

 not incompatible. The influence of salinity on the amount of yolk 

 simply gives a more intimate explanation of the assumption of Sollas. 29 

 Entry into fresh water is for the most part preceded by a sojourn in 



