390 



CONCHIFERA. 



Fig. 180. 



like that of the oyster, respiration would have been impossible 

 under the circumstances in which they live ; but, by the modifi- 

 cation 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 ex- 

 crement ejected through the 

 other (fig. 180). 



Whoever watches these 

 syphoniferous bivalves in a 

 living state will readily ap- 

 preciate the importance of 

 the pallial prolongations 

 forming this tubular appa- 

 ratus ; especially if minute 

 floating particles are placed 

 in the water wherein they 

 are confined. It will then 

 be perceived that powerful 

 currents are perpetually 

 rushing through the extre- 

 mities of each syphon, caused 

 by the rapid action of cilia 

 placed within ; and the 

 streams thus produced not 

 only form a provision for 

 constantly changing the water in which the branchiae (fig. 180, g) 

 are immersed, but forcibly convey floating molecules to the aper- 

 ture of the mouth, which is situated in the position indicated in 

 the figure by the letter h, and thus supply abundance of nutritive 

 materials that could, apparently, in animals so destitute of prehen- 

 sile organs, have been procured by no other contrivance.* 



The last family of this class includes those species which, like the 

 Pholas and Teredo, bore in stone or wood ; or, like the Solen, pene- 

 trate deeply into the sand. In such, the mantle is prolonged into 

 terminal tubes of great length, and their shells remain always open 



* The parts represented in the above figure (fig. 180) which are not particularly 

 pointed out in the text are, the anterior adductor muscle, c ; the posterior adductor 

 muscle, d ; the elastic ligament of the hinge, e ; and the largely developed foot,/. 



