50 Dr. T. Williams on the Mechanism of Aquatic 



are carried in the direction of the mouth and tried and tasted 

 by the palpi (fig. 6, i, i). Those which are acceptable are 

 swallowed; those which are unpalatable are carried completely 

 out of the cavity indifferently by any one of its openings, lest 

 they should again pass over thebranchia?. So incomparably ad- 

 justed are the cilia which render the gills a wondrous spectacle 

 of infinitesimal currents, so precise and fore-ordained are the 

 directions in which they move, that the act which sieves the 

 food from the water drives also that water from the recipient 

 into the refuse chamber (fig. 6, /; PL II. fig. 12, h), through the 

 meshes circumscribed by the branchial vessels. This stage of the 

 respiratory process is strictly involuntary. It is governed by in- 

 violable organic laws, not volitional caprice. It is to the mollusk 

 what the insensible involuntary physical exos- and endosmose of 

 gases in the ultimate air-cells of the lungs are to the mammal. 

 It differs physically and physiologically from the act in which 

 water is drawn into the cavity of the mantle, -as strikingly as the 

 thoracic movements of respiration in Man differ from the ulti- 

 mate process. Thus, whatever may be the number, size, or 

 prominence of the openings* of the mantle, the functions of the 

 great ventral chamber remain unchanged. They are and must 

 be in every instance, under all general mutations of character, 

 those of a reservoir from which food is drawn to the mouth and 

 the aerating element to the branchiae. 



The second great cavity (PI. I. fig. 6, /; fig. 7, c. PI. II. 

 fig. 8, d; fig. 9, e ; fig. 10, c, c' ; fig. 12, h), lying to the left, 

 dorsal, or "the hinge" side of the imaginary line formerly 

 defined, remains now to be described. It is limited ventradly 

 by the branchiae, dorsadly by the hinge, and posteriorly by the 

 ex-current siphon. 



The anal chamber does not in all genera communicate openly 

 and directly with the interlamellar passages. The former really 

 arises in the latter, when only one of the proximal borders of the 

 branchial lamellae is attached to the side of the visceral mass ; 

 the grooves, running antero-posteriorly and parallel with the 

 length of the gill, and situated between its proximal borders and 



* I would beg here to refer the student to the interesting papers of Mr. 

 Hancock, in the ' Annals and Magazine of Natural History ' during the 

 years 1852 and 1 853, for an account of the collateral openings which in some 

 genera occur in the mantle. In Chamostrea albida, in addition to the nor- 

 mal siphonal orifices and pedal and ventral gapes, he describes another of 

 minute size, which is situated under the lower siphon. A similar aperture 

 exists in other Lamellibranchiates. Mr. Hancock has observed it in Lutraria, 

 Cochlodesma, Panopcea and Myochama and Prof. Owen in Pholadomya. It 

 is clear from the explanation given in the text, that these secondary aper- 

 tures are really secondary iu meaning. They do not in the least affect the 

 physiological character of the cavity- 



