Organs employed in the Classification of the Mollusca. 389 



opposition to the Linnsean system founded chiefly on the ana- 

 tomical researches of Ray. The Cetacea, but chiefly the Reptiles 

 (Anguis, Bipes, Sec), afford striking examples of the insignifi- 

 cance of the locomotive organs as a base for the primary divi- 

 sions. 



The secondary divisions (orders) of Cuvier were founded on 

 the respiratory organs. Respiration is indispensable for the life 

 of all organic beings ; but special organs for this purpose are not 

 always necessary. Respiration of some importance takes place 

 through the skin, even in the higher Vertebrata, and can, in the 

 lower Vertebrata (e. g. Batrachians), temporarily replace that 

 through the lungs : it is quite sufficient for many Mollusca. 



The Vertebrata have two kinds of respiratory organs — lungs, 

 and gills — which, according to J. Miiller, are not homologous, 

 as they can be found together in the same individual, although 

 not always exactly performing the same function — for instance, 

 the Batrachians and the foetus of the higher Vertebrata. In 

 fishes the lungs are reduced to a swimming-bladder, the walls 

 of which are provided with some bloodvessels, making it serve 

 as a secondary, but very imperfect, respiratory organ comparable 

 to the lung-sac of the Pulmonata. That this organ, even among 

 the Mollusca, cannot be homologous with the gills, as advanced 

 by Prof. Agassiz *, is proved in Ampullaria and perhaps Onchis, 

 each of which has lung-sacs simultaneously with true gills. 

 Many Mollusca (as Cyclostoma, Neritina, and Littorina), which 

 are undoubtedly provided with gills, live always or nearly always 

 in the air, probably having the power of keeping their gills moist, 

 like the Land-crabs and several fishes (e.g. Anabas scandens) . 

 Whether the vena branchialis and vena pulmonalis are identical 

 is at least not yet proved. In Mollusca not requiring a hard 

 covering for their protection, respiration takes place through the 

 skin ; but when the skin is thickened, or a shell developed, a 

 respiratory organ becomes necessary. The use of the gills is to 

 produce, by the complication of a part of the skin, a surface 

 corresponding with the area made impenetrable to the oxygen 

 of the surrounding medium (air or water). The effect of this 

 dermal gland is increased by vibratile cilia, producing a swifter 

 circulation of the oxygeniferous medium. If the branchial sac 

 is very deep, the circulation of the water is effected by a siphon 

 acting like a chimney-pipe, often assisted by muscular contrac- 

 tions (Cephalopoda, Acephala). The larger the shell is in pro- 

 portion to the uncovered parts of the animal, the more compli- 

 cated and compressed are the gills. 



Several of the internal glands are subject to the same change. 



* "On the Circulation of Fluids in Insects," Proc. Bost. Soc. 1850, 

 p. 237. 



