382 INTRODUCTION TO CONCHOLOGY. 



of the hranclim, or breathing-organs^ differ materially from 

 such as pertain to the Lamellibranchs : they consist of 

 numerous beautiful veins and arteries, incorporated within 

 the substance of the two lobes of the mantle. The calci- 

 fying organ of the Bracliiopocla has, therefore, a double 

 function : it not only secretes calcareous mucus for the for- 

 mation of the shell, but is made subservient to the circula- 

 tion of aerated fluid. Professor Owen has well observed, 

 that, " in this profuse distribution of vessels over a plain 

 membranaceous surface, we perceive the simplest construction 

 of the water-hreathing organ, presenting a beautiful analogy 

 with the elementary forms of the air-hreat7ii7ig organ, as 

 shown in the Helices and Bulimi, with their immerous 

 relatives.''^ 



The muscular system in these singular moUusks is also 

 most appropriate, though complex ; the Lingulm and Orli- 

 cul(E are provided with three pair of muscles, and the Tere- 

 bratidce with four. The large ones are destined to open 

 and close the valves in the absence of a hinge filament, an'^ 

 the smaller assist in sliding one valve over the other for tne 

 admission of water. 



With regard to the situation which the Brachiopodouc- 

 mollusca should hold among their brethren, naturalists hav-^ 



