38 



HISTORY OF FARM 



limpet-shaped. Two of these are shown in 

 Figure 19. In all of them flaring margins 

 of the body fit down closely to the stone and 

 deflect the water, so that it presses them 

 against their support. 

 „ .„ _ In still water the deep pools are the 



Fig. 19. Two Jr- f 



insect larvae that special homc of the larger fishes. We shall 



stick to stones in 



rapid water: a, the rctum to them m the next study. In the 



flat riffle- beetle i • , ■ ■, 



(Psefhenusiacontei) shoalcr parts and m the midst of the aquatic 



and 6, the net- . ^ 



veined midge (Bfe- vegctatiou are the lesser fishes and many other 



pharocera). 



familiar vertebrates, frogs and their tadpoles, 

 salamanders, turtles, etc., of uncertain occurrence. Much 

 more generally distributed and constantly present are a 

 few molluscs and crustaceans, such as are shown in Figure 

 20. There are a few adult insects and many insects in im- 

 mature stages (Figs. 21,22 and 23). Some help toward the 

 recognition of these may be had from the table on pages 40 

 and 41, which contains brief hints, also, of the situation they 

 occupy in the water and the role they play in the food 

 consumption. 



There are leeches, and fresh water sponges and bryozoans 

 and a host of lesser forms of many^groups, mostly too smalljto 



CPVST^C£'4^S 



Fig. 20. Some common crustaceans and molluscs: — crawfish, with the asellus at 

 the left and the scud (.Gamtnarus) at the right: also, a mussel and two snails: Limneat 

 on the left, and Planorbis on the right. 



