RESPIRATION. 261 



that had nearly slipt from my remembrance. It has, I 

 think, been very generally received as an axiom in physi- 

 ology that the power of locomotion bears a ratio to the 

 perfection of the respiratory organs : the more perfect the 

 latter, and the greater their capability of submitting the 

 blood to the action of the oxygenating medium, the more 

 vivacious and agile the animal in its movements. Thus, the 

 cuttle-fish, with their laminated and highly-developed gills, 

 move in the bosom of the ocean with quickness and vigour ; 

 and the pulmoniferous and pectinibranchial mollusks have 

 been advantageously contrasted with the sedentary bivalves 

 and the ascidians. But the latter comparison is surely an 

 unfortunate one, for I know no mollusks in which the gills 

 are so large in proportion to the body as in the fixed oyster 

 and anchored mussel. Indeed, in regard to the mollusca the 

 axiom will not hold good. Some of the bivalves, such as 

 the Cyclas, move with little less rapidity than the Limneus 

 and pulmoniferous Gasteropods ; and the water-breathing 

 Rissose are more quick of foot by far than the slugs and 

 snails which breathe uncombined air. Even in the same 

 tribe and family there are such differences in respect of 

 speed that it seems impossible to ascribe much influence on 

 it to any formation of the gills. Thus, the Buccinum un- 

 datum is preeminently tardigrade, but its near ally the 

 Nassa maculata is quick and active ; and similar examples 

 may be easily pointed out. 



Nor do I believe that there is any connection between 

 any structural peculiarities of the respiratory organs and the 

 varying depths in which the mollusca do live. In his mas- 

 terly anatomy of the Brachiopoda, Professor Owen would 

 seem, perhaps, to intimate the contrary. He says, that to 

 the Ligula a respiratory apparatus more complex and obvious 

 than that of the Terebratula, was indispensable, because the 

 former lives more commonly near the surface, and where it 

 must meet with a greater variety and abundance of animal 

 nutriment than can be found in those abysses in which Tere- 

 bratula is destined to reside. He continues: "The respi- 

 ration, indeed, as well as the nutrition of animals living 

 beneath a pressure of from sixty to ninety fathoms of 

 sea-water, are subjects of peculiar interest, and prepare the 

 mind to contemplate with less surprise the wonderful com- 

 plexity exhibited in the minutest parts of the frame of 

 these diminutive creatures. In the stillness pervading these 

 abysses they can only maintain existence by exciting a per- 

 petual current around them, in order to dissipate the water 

 already loaded with their effete particles, and bring within 



