208 CIRCULATION IN GASTEROPODA. 



" The Cephalopods of the higher order have a power of 

 locomotion superior to all other mollusks, and can vary their 

 elevation in the water at will. The Pearly Nautilus also, 

 though in general dwelling in the deep, has the power of 

 rising and floating on the surface, as appears from the testi- 

 mony of Rumphius, and the circumstances connected with 

 the capture of the present specimen. These changes of 

 position must obviously produce great alterations in the 

 degree of pressure which the animals have to sustain from 

 the surrounding medium ; and the fluids contained in their 

 sanguiferous system must, of course, suffer considerable and 

 corresponding variations of expansion. We must suppose 

 also, that their respiration, or the transmission of the blood 

 through the gills, will be more or less rapid, both according 

 to the distance from the surface at which they breathe, and 

 the degree of muscular effort that may at any time have 

 been expended. 



" In other classes of animals subject to the same muta- 

 tions of surrounding pressure, various structures have been 

 recognised as accommodating the sanguiferous system to 

 these changes ; as, for example, the extraordinary rete mira- 

 bile in the intercostal spaces of the Cetacea, and the varied 

 muscular and elastic powers connected with the branchial 

 artery of fish, which, according to Sir Everard Home, bear 

 a relation to their powers of descending to great depths. 

 The auricle also in fish, and the capacious venous sinuses 

 which terminate in it, must afford convenient receptacles to 

 the blood when in a state of expansion, or prevented by any 

 cause from flowing freely through the gills ; and the valvular 

 structures for obviating the regurgitation of the contents of 

 the ventricle into the auricle, or of the auricle into the 

 sinus, are more complete in this than in any other class of 

 vertebrate animals. But the branchial ventricle, in those 

 Cephalopods which possess it, is unprovided with an auricle ; 

 and the Pearly Nautilus, if we except the follicles appended 

 to the vessels passing to the gills, has no receptacle con- 

 nected with the vascular system adapted to the uses above 

 mentioned. I am therefore induced to believe that these 

 follicles relieve the vascular system, by affording a temporary 

 receptacle for the blood whenever it accumulates in the 

 vessels, either from the effects of a general expansion, or of 

 a partial impediment in its course through the respiratory 

 organs, and that they serve to regulate the quantity of blood 

 sent to these organs." * 



* Memoir on the Pearly Nautilus, 33, 34. 



