DISTRIBUTION AND ADJUSTMENT 105 



may be platelike structures, as in the damsel-fly nymph, or they 

 may be filamentous, as in the larvae of stone fly, May fly, or 

 whirligig beetle (Fig. 212). In the nymph of the common dragon 

 fly respiration is accomplished through the wall of the large 

 intestine, and a special chamber for the purpose is attached to 

 this organ, the water flowing into it through the anal opening. 



Snails, clams, and fish have largely remained water breathers, 

 as have also most of the crustaceans. Some of the latter like 

 the land sow bug have taken to damp situations on land, as under 

 old logs and under the bark of stumps. The common chimney 

 crayfishes live much of the time out of the water near the tops 

 of their burrows, still breathing however by gills (see Fig. 379). 

 Many snails have come to be land dwellers, always in damp 

 situations, though, but still they breathe by means of a lung as 

 do even some of the pond forms. Locally we have no fish that 

 live out of the water, but there are such that run around on the 

 mud banks or even climb logs and inclined tree trunks in search 

 of insects; and in some the African lung fish, for example- 

 respiration is accomplished in the air by means of the modified 

 swim bladder, an organ that in most fish is a float, but that 

 serves in this one the purpose of a primitive lung. 



We do have examples in abundance of the higher vertebrates 

 that have taken to the water as a hunting ground and must 

 modify their air breathing to suit the exigencies of the case. The 

 frogs, turtles, some snakes, such birds as the grebes, and mammals 

 like the muskrat are cases in point. The muskrat has ring 

 muscles about its nostrils by the contraction of which he can 

 close these openings when he dives. The bones of grebes have 

 spongy interiors as indeed -do those of all birds. The lung 

 cavities connect with these porous portions so the air-supply 

 that can be taken below water is increased. The frog spends 

 its winter entirely submerged, hibernating in the mud at the 

 bottom of the pond. Its mouth and nostrils are then closed 

 air tight and what respiration goes on is accomplished through 

 the skin, a return to the primitive method. 



