RESPIRATORY TUBES. 503 



upon the already-mentioned habits of this extensive division of the Coir- 

 Fig. 250. 



Mactra : a, foot ; 6, c, siphons formed by the mantle. 



Kg. 251. 



CHIFEEA, and consider how, 

 by means of their largely- 

 developed foot, they burrow 

 into the sand or mud of the 

 shore. Had their mantle 

 been open, like that of the 

 Oyster, respiration would 

 have been impossible under 

 the circumstances in which 

 they live; but, by the 

 modification of structure 

 thus provided, their tubes 

 being prolonged to the 

 mouth of the excavation 

 wherein they reside, water 

 is freely admitted to the 

 branchiae through one of 

 the passages so formed, and 

 excrement ejected through 

 the other (fig. 251). 



(1324.) Whoever watches 

 these siphoniferous bivalves 

 in a living state will readily 

 appreciate the importance 

 of the pallial prolongations 

 forming this tubular ap- 

 paratus ; especially if mi- 

 nute floating particles are 

 placed in the water wherein 

 they are confined. It will then be perceived that powerful currents 



Mactra : a, b, siphons ; e, hinge ; c, d, adductor muscles ; 

 /, foot. 



