BORING-BIVALVES STRUCTURE OF SIPHONS. 487 



tions in the structure of the animal. Throughout 

 the various races of Oysters and Scallops, as we have 

 already had an opportunity of remarking, the margins 

 of the mantle being free and unattached, admission is 

 at all times afforded to the surrounding water ; and 

 were creatures so circumstanced to be buried in sand 

 or mud, they would speedily have their delicately- 

 ciliated branchial fringes so completely clogged up 

 that they would immediately perish. 



In the boring bivalves, therefore, the mantle no 

 longer offers the same simple arrangement ; but the 

 two sides becoming gradually more and more united 

 along their edges, the bodies of these mollusks are 

 by degrees entirely enclosed, as though shut up in a 

 sac, so that the external element is denied admis- 

 sion, except by two membranous tubes, sometimes 

 of considerable length, called siphons, through which 

 the water is conveyed to the gills, and effete materials 

 are expelled into the surrounding ocean. 



We at once perceive the use of the tubular arrange- 

 ment here referred to : had the mantle been open, 

 like that of the Scallop, respiration would have been 

 impossible under the circumstances in which these 

 creatures 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 again returned through 

 the other in a vitiated condition. Whoever watches 

 one of these siphoniferous bivalves in a living state, 

 will readily appreciate the importance of this siphonal 

 apparatus, especially if minute floating particles are 



