302 PRINCIPLES OF ANIMAL BIOLOGY 



bottom is one in which roots can take hold (clayey or sandy), and a 

 rich animal population finds shelter, support, and oxygen among the 

 plants. Where plants are lacking merely because shifting of the sand 

 prevents their rooting, numerous animals (clams, worms, sea cucumbers, 

 crabs) burrow in the bottom material and feed on remains of seaweeds and 

 animals. Gravel bottom is practically without life, because movements 

 of the pebbles in strong wave action destroy living things. Animals of 

 this coastal area depend for food on the organic matter (largely dead 

 bodies, including plant remains) brought in by rivers or produced in the 

 coastal area itself. 



Between low and high tide there are fewer animals, yet some are 

 able to endure the twice-a-day uncovering, exposure to the heat of the 

 sun in summer and temperature extremes in winter, and dilution of the 



Fig. 266. — A burrowing animal between tide lines; the clam Scrobicularia. 



water by rains. The clam Scrobicularia (Fig. 2G6) burrows in the sand 

 and with its long inhalent siphon explores the surface around it for 

 food-bearing water. 



The ocean bottom below the effective penetration of light is less well 

 populated. The organic remnants which serve as food here are the 

 decaying bodies of swimming and floating animals and plants which 

 settle down from above. The ooze thus formed has a pasty consistency. 

 The most abundant bottom animals of the deep sea are sea cucumbers. 

 Others are Crustacea (amphipods and isopods), hydroids, sponges, clams, 

 and worms. In general, deep-sea animals are smaller than their relatives 

 near the surface. Also they may be more delicately constructed (fragile 

 skeletons, thinner shells) liecause there is little motion of the water. 



Animals of the Open Ocean. — Organisms of open water either swim 

 or passiv-ely float. Floating life must have some way of reducing its 

 specific gravity, since protoplasm itself is heavier even than salt water. 

 One way is to take up much water, without the salt, into the tissues. 

 Other ways are to develop fat, or gas chambers like those of the siphono- 



