176 FRESHWATER FAUNA 



twelve miles above its mouth ; a razor shell (Novaculina 

 gangeticus) occurs in the Ganges ; a Tellina (Galatea} in 

 rivers in Africa ; and a Mactra (Gnatlwdon) in fresh water 

 about the Gulf of Mexico. 



These instances might be added to, but without greatly 

 diminishing the contrast between the marine and fresh- 

 water faunas. They show that climate has its effect, but 

 on the whole this is perhaps not so great as we might 

 have expected. 



The composition of the water and the character of the 

 climate, these are both barriers to the invasion of our 

 rivers, but they are far from insurmountable. If these 

 were all, w r e might expect to find numerous marine 

 animals, under the pressure of existence, enterprisingly 

 working their way along the shores of the numerous 

 streams which open along the coast, and every river 

 characterised by a modified marine fauna derived from 

 the neighbourhood of its mouth. 



So far from this being generally the case, freshwater 

 forms of life are, as a rule, well distinguished from the 

 marine, and many of them maintain their distinctive 

 characters over the greater part of the world, and far 

 back into geologic time. 



Some other impediment, or more than one, must exist ; 

 and one possibly one of the most important is to be 

 found in the movement of the river water, constantly 

 directed towards the sea, always advancing, as it were, 

 against a possible invasion. The effect is probably in- 

 direct. The prevailing method by which marine animals 

 obtain a distribution over extensive areas is by means of 

 free-swimming larvae. The sedentary oyster, although 

 unable when adult to leave the oyster bed, passes its 

 early life in a round of dizzy movement (Fig. 54*). 



* This figure represents the larval form of Dreissena, but will 

 serve to illustrate the general appearance of a larval oyster. 



