38 NATURAL HISTORY OF THE 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. 
Fic. 19. Two i still water the deep pools are the 
insect larvae that special home of the larger fishes. We shall 
stick to stones in 
rapid water: a, the return to them in the next study. In the 
at riffle- beetle 
ee. econ- shoaler parts and in the midst of the aquatic 
peed Seer (Ble- vegetation are the lesser fishesand many other 
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 (fig. 21) and many insects 
in immature stages (figs. 22, 23) and24. 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 thefood 
consumption. 
There are leeches, and fresh-water sponges and bryozoans, 
and a host of lesser forms of many groups, mostly too small to 
CRUSTACEANS 
mussel 
Fic. 20. Some common crustaceans and molluscs: crawfish, with the asellus at 
the left and the scud (Gammarus) at the right;—also, a mussel and two snails; 
(Limnea, on the left, and Planorbis on the right). 
