252 Adjustment to Conditions of Aquatic Life 



Avoidance of silt — Gills are essentially thin- walled 

 expansions of the body, that provide increased surface 

 for contact with the water, and thus promote that 

 exchange of gases which w^e call respiration. Gills 

 usually develop on the outside of the body; for it is 



only in contact with the water 

 that they can serve their func- 

 tion. In most animals that live 

 in clear waters they are freely 

 exposed upon the outside; but 

 in animals that live on soft 

 muddy bottoms they are with- 

 drawn into protected chambers 

 (or, rather, sheltered by the 

 outgrowth of surrounding parts) 

 and fresh water is passed to 

 them thro strainers. Thus the 

 gills of a craw^fish occupy capa- 

 cious gill chambers at the sides 

 of the thorax, and water is 

 admitted to them thro a set 

 of marginal strainers. The gills 

 of fresh -water mussels are located 

 at the rear of the foot within the 

 inclosure of valves and mantle, 

 and water is passed to and from 

 them thro the siphons. The gills 

 of dragonfly nymphs are located 

 on the inner walls of a rectal 

 respiratory chamber, and water to cover them is slowly 

 drawn in thro a complicated strainer that guards the 

 anal aperture, and then suddenly expelled thro the 

 same opening, the valves swinging freely outward. 



There is probably no better illustration of parallel 

 adaptation for silt avoidance than that furnished by the 



Fig 154. The abdomen of 

 Asellus, inverted, showing 

 gill packets. 



