BREATHING OF BIVALVES 113 



base of the foot-jaws and exhales it beneath the 

 eyes. When sunk in sand it performs the opposite 

 movement, and drives the water out at the opening 

 through which it had previously inhaled it. The 

 masked crab, which burrows deeply into the sand, 

 possesses a special breathing tube, through which it 

 can inhale fresh draughts of water. Its antennae are 

 brought together by interlocking hairs, so producing 

 the tube, and the water then passes backward over the 

 gills and out amongst the feet (fig. 19, F). 



The burrowing bivalves razor-shells, rainbow- 

 shells, cockles, and the like provide themselves with 

 a breathing siphon. To escape their numerous enemies 

 and to evade the digging action of the sea, that threatens 

 to evict and pulverise them, they possess a flattened 

 shell, which easily passes through the sand as the 

 spade-like foot hauls them into the burrow. But 

 without some means of breathing surface-water these 

 animals would be speedily choked, and their delicate 

 gills lacerated by the flinty particles around them. 

 Accordingly we find they have developed a long, 

 bent siphon, the original fringe of their double tube 

 drawn out to twice the length of the animal. Through 

 this double tube the bivalves can reach up to the 

 surface-water and tap a supply unmixed with sand. 

 The current runs down one tube, bathes the delicate 

 plate-like gills, and after passing through the body rises 

 again to the surface, where, to avoid contaminating the 

 supply, the outlet tube is turned away. When alarmed 

 the animal withdraws its tubes into the shell, in which 



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