188 



MARINE ANIMALS 



makes possible an estimation of distance, which is of the greatest im- 

 portance in securing their prey. 6 The air bladder is absent or reduced 

 in the fishes of this habitat. Trachinus and Uranoscopus bury them- 

 selves deeply. The great majority of the flatfishes lie upon the bottom, 

 adapting their coloration to that of their background by means of their 

 power of color change. They stir up the sand with a few undulating 

 motions, and when this settles it hides the outlines of their bodies. 

 Among rays, the tropical Trygonidae bury themselves in sand; other- 



Fig. 28. — Pirula sp. (a), with broad flattened foot, living on loose bottom, 

 contrasted with Rostellaria curia, (b), with narrow foot, from solid bottom at 

 45-m. depth. After Hornell. 



wise sharks and rays for the most part lie on the surface, protected by 

 their spiny coat and rendered indistinct by their coloration. 



Among the squids, Sepia tends to keep itself slightly embedded in 

 sand or mud, adjusting its coloration to that of the bottom material. 



The number of forms living on the surface of the sand is small in 

 contrast to the hidden inhabitants of sand bottom. Numerous brittle 

 stars and some starfishes have this habit, and in them the tube feet are 

 rounded instead of ending in a sucking disk. A number of carnivorous 

 snails, such as Buccinum and Nassa, creep over the surface of the sand. 

 Such snails, on account of the looseness of the bottom, require a broad 

 foot, a contrast with snails in rocky habitats, especially in quiet water, 

 which require only a small foot (Fig. 28). A number of crabs, such as 

 Portunus, Hyas, and Inachus, live on the sand surface, and this is the 

 typical habitat of the hermit crabs, which often occur in great numbers. 



