92 FRESH-WATER ANIMALS 



daylight only penetrates to a limited depth in water, 

 and it has been shown that below 25 fathoms 

 photographic paper is not acted on by daylight. 

 It is also found that 25 fathoms is about the depth 

 to which the pelagic Entomostraca descend during 

 bright sunshine. The object of so descending is 

 apparently to utilise the light, so as to range during 

 the four and twenty hours over their whole hunting 

 ground for food. Were they to remain at the 

 surface in the daytime they would lose their sole 

 chance of obtaining food from the greater depths. 

 It is for this reason again that the eyes are of such 

 great size, for it is clear that forms with larger and 

 more perfect eyes would have a better chance in 

 the pursuit of food than those less well equipped ; 

 and hence by the action of natural selection, the 

 large-eyed forms would survive, and any further 

 improvement of the eyes would be preserved and 

 transmitted to their descendants. 



Turning now to the Mollusca, one of the best 

 known and interesting fresh-water forms is the 

 common river mussel, Anodonia cygnea. Like 

 bivalves in general, Anodonta produces eggs of 

 small size ; however, these are not immediately 

 passed from the body of the parent, but are 

 transferred to the outer gills where they remain for 

 some time, and where they pass through all the 

 early stages of their development. 



The gill consists of a couple of laminae fused 

 together along their ventral edges ; each lamina is 

 not a continuous membrane, but a trellis-work 

 composed of very numerous vertical bars, crossed 

 and connected together by a smaller number of 



