452 CEPHALOPODA. 



by this arrangement a very extensive surface is obtained, over 

 which the blood is diffused for the purpose of respiration. The 

 respiratory apparatus is lodged within the visceral sac, but sepa- 

 rated from the other viscera by a membranous septum (Jig. 211, t) ; 

 so that a distinct chamber is formed to contain the branchiae, where- 

 unto the water is freely admitted ; the surrounding element being 

 alternately drawn into the branchial cavity by the action of its 

 muscular walls, through a valvular aperture provided for the pur- 

 pose, and again expelled in powerful streams through the orifice of 

 the funnel. Such, indeed, is the force with which the water is 

 ejaculated through the funnel, that it not only serves to expel from 

 the body excrementitious matter derived from the termination of 

 the rectum (Jig. 211, s), which opens into the respiratory cavity, 

 but becomes one of the ordinary agents in locomotion. This mode 

 of progression, although in fact common to most of the Cepha- 

 lopod tribes, is remarkably exemplified in the Argonaut, which, 

 instead of navigating the surface of the sea, as has been already 

 stated, simply darts itself from place to place by sudden and oft- 

 repeated jets thus violently spouted forth ; while with its arms 

 stretched out and closely approximated, and its vela tightly ex- 

 panded over the outward surface of its delicate shell, it shoots 

 backwards like an arrow through the water. 



(495.) Separated from the chamber in which the branchiae are 

 lodged, by the. membranous partition already mentioned (Jig. 

 11, ), and likewise distinct from the peritoneum containing 

 the viscera, is a considerable cavity, divided by a membranous 

 partition into two compartments, wherein are placed the great 

 trunks of the venous system (d, d). These chambers, named 

 by Cuvier* the " great venous cavities," are very remarkable ; in 

 as much as, although they contain the vena cava, which here pre- 

 sent a truly anomalous structure, they are lined with a mucous 

 membrane derived from the branchial chamber, with which they 

 are in free communication, and from whence the external element 

 has free admission to their interior. 



It is in this " great venous cavity" called by Professor Owen 

 the " pericardium" that, in the Pearly Nautilus, the syphon 

 which traverses the partitions of its camerated shell (Jig. 205) 

 terminates ; and the reader will now perceive by what mechanism 

 water received from the branchial chamber may, in that animal, 

 be injected into its partitioned shell for the purpose already referred 

 to (479). 



* M6moire sur le Poulpe. 



