OYSTERS. 153 



out their dwellings in stone or wood, and thus enjoy the pro- 

 tection of a retrenched camp, can do with a thin and brittle, 

 or even with a mere rudimentary shell. The Solens, which at 

 the least alarm are able to sink rapidly into the sand, and 

 indeed very rarely quit the hole in which they are comfortably 



Razor shells. 

 a foot. b c siphons. 



ensconced, require no closely fitting valves ; but the oysters, 

 which have no external fortress to retire to, would be in a very 

 bad predicament if they could not entirely conceal themselves 

 within their thick shells, and keep them closed by strong mus- 

 cular contraction. Even so,, they are exposed to many persecu- 

 tions, not only of man, or of the oyster-catcher who dexterously 

 opens them with his strong beak, or of the star-fish who clutches 

 them in a murderous embrace, but of a whole host of far more 

 minute though not less formidable enemies. Small annelides, 

 boring through the shell, attack them at all points, and though 

 they endeavour to parry these assaults by fresh depositions of 

 pearly matter, their strength is sorely taxed in the struggle. 

 Soon parasitic sponges establish themselves in the holes pierced 

 by the annelides, and eating deeper and deeper into the shell, 

 at length entirely exhaust the poor victim, who sinking under 

 his accumulated misfortunes gives up all further resistance, and, 

 breathing his last, allows the enemy to penetrate into his 

 gaping valves and to make a delicious meal of his remains. 



The nervous system of the acephalous mollusca consists of 

 three pair of ganglia, which send their filaments to the respective 

 organs. The sense of touch appears to reside in every part of 

 the body and of the mantle ; that of hearing is lodged in the 

 foot, and consists of two globular capsulse inclosing a limpid 

 fluid, and a calcareous body or otolithe suspended from the 

 filaments of the auditory nerve, to which all its vibrations are 

 instantly communicated. 



