254 Adjustment to Conditions of Aquatic Life 



enough, indeed, to give them a bright red color. This 

 substance has a great capacity for gathering up oxygen 

 where the supply is scanty, and of yielding it over 

 to the tissues as needed. True worms that burrow in 

 deep mud, and Tubifex (see fig. 83 on p. 174) that bur- 

 rows less deeply and the larger bright red tube making 

 larvae of midges known as ''blood worms" (see fig. 236 

 on p. 393) are examples. Since these forms live in the 

 softest bottoms, where the supply of oxygen is poorest, 

 where few other forms are able to endure the conditions, 

 their way of getting on must be of considerable efficiency. 



II 



Burrowing — The ground beneath the water offers 

 protection to any creature that can enter it ; protection 

 from observation to a bottom sprawler, that lies littered 

 over with fallen silt; protection from attack about in 

 proportion to its hardness, to anything that can bur- 

 row. 



Animals differ much in their burrowing habits and in 

 the depth to which they penetrate the bottom. Many 

 mussels and snails burrow very shallowly, push- 

 ing their way along beneath the surface, the soft foot 

 covered, the hard shell-armored back exposed. The 

 nymphs of Gomphine dragonflies (fig. 116 on p. 209) 

 burrow along beneath the bottom with only the tip 

 of the abdomen exposed at the surface of the mud. 

 Other insect larvae descend more deeply into burrows 

 which remain open to the water above : while horsefly 

 larvae and certain worms descend deeply into soft mud. 



The two principal methods by which animals open 

 passageways thro the bottom are (i) by digging, and 

 (2) by squeezing thro. Digging is the method most 

 familiar to us, it being commonly used by terrestrial 

 animals. Squeezing thro is the commonest method of 

 aquatic burro wers. 



