OF RESPIRATION. 219 



pac ; the inhaled currents are made to traverse the body by 

 the cilia, encircling the afferent aperture, and developed on the 

 surface of the branchial membrane. 



In the CONCHIFERA the mantle presents two orifices, the 

 one for the entrance and the other for the exit of the water 

 from the branchial cavity. In the oyster (fig. 1/6), the bran- 

 chiae form four leaflets (h, K), attached by their contiguous 

 upper margins, and free below ; they consist of innumerable 

 elongated filaments, covered by a delicate membrane, on 

 which a rete of capillary blood-vessels is spread ; vibratile 

 cilia are developed on the surface of this membrane, as well 

 as on that of the branchial cavity, by which currents of water 

 are made to traverse the respiratory organs in a determinate 

 direction ; in the conchifera, burrowing in rocks, sand and 

 mud, the branchiae are greatly elongated, and the mantle is 

 prolonged into tubes, for conducting water into the palleal 

 cavity. The vibratile cilia are of large size in Mytilus and 

 Anodon, covering the entire surface of the branchial filaments, 

 and lining all parts of the respiratory cavity ; a small portion 

 of the branchiae, detached from the living animal, is seen to 

 row itself, like an animalcule, through the water, by the 

 motion of its cilia. 



Nearly all the GASTEKOPODA respire by branchiae, which, in 

 most of the naked marine species, are in the form of tufts, 

 fans, or combs, variously disposed on the surface of the body, 

 and in the testaceous kinds are concealed under a fold of the 

 mantle. In the Doris (fig. 221) the branchiae (e) form elegant 

 ramose tufts, disposed around the anal opening ; in Thethys 

 they are composed of two dorsal rows of alternately tufted and 

 crested organs. In Aplysia (fig. 177) they occupy the right 

 side of the body, and are protected by a delicate peUucid 

 shell. In the numerous pecteni-branchiate gasteropods, as the 

 Paludina (fig. 35), inhabiting univalve turbinated shells, the 

 branchiae ((/} are placed under an extended fold of the mantle, 

 and in many of the carnivorous genera the water is con- 

 ducted into the branchial chamber, through a muscular si- 

 phuncle, lodged in a canal of the shell, and flowing over the 

 surface of the filamentary gills, by the vibrations of the cilia, 

 is discharged through an opening in the palleal cavity, carrying 

 with it the excreted materials from the glands and intestinal 

 canal. 



