274 CEPHALOPODS BIVALVES. 



tions of them in the remaining Gasteropoda have been 

 already noticed slightly in the outline I gave you, in an 

 early letter, of the Cuvierian system : but there is one family 

 which, because of its greatness, may not be passed over in 

 this place, this is the Pectinibranches, an order that in- 

 cludes almost all the marine turreted and convolute * shells, 

 and a few which are found in fresh water. In it the bran- 

 chial cavity has a position similar to that in the Pulmonifera, 

 on the upper and fore part of the back ; to wit, where it 

 is protected by the body of the shell ; but its walls are 

 not smooth and even, like those of the pulmonary cavity, 

 but folded into neat and regular plaits or ridges, that lie 

 parallel to one another, like the teeth of a comb (whence 

 the name Pectinibranches), and often part on each side from 

 a central stalk formed by the trunks of the bloodvessels, 

 in the same manner that the barbs of the web of a quill 

 depart from the shaft. The water obtains ingress to this 

 cavity, in such Pectinibranchia as inhabit shells with entire 

 apertures, by a large slit on the side above the collar ; and 

 in shells with interrupted or beaked apertures, by an im- 

 perfect siphon that lies in the canal or emargination, and 

 that is formed by a prolongation and duplicature of the 

 cloak. 



The gills of the Cephalopods are placed within their 

 muscular sac, to which they are attached by their bases 

 only in the Nautilus, but in all the naked genera a thin 

 fibrous membrane connects the fleshy stem of each gill to 

 the contiguous surface. In the Nautilus there is another 

 remarkable peculiarity: it has four branchiae, a larger and 

 smaller one on each side, while all other Cephalopods are 

 dibranchiates. The branchiae present an elongated pyra- 

 midal figure with their apices directed forwards. " They 

 are composed of a number of triangular vascular laminae 

 extending transversely from each side of a central fleshy 

 stem, having an alternate disposition : each lamina is com- 

 posed of smaller transverse laminae, which are again simi- 

 larly subdivided; the entire gill thus exhibiting the structure 

 called by botanists " tripinnate," by which an extensive 

 surface is afforded for the minute division of the branchial 

 vessels." f 



* Sir E. Home asserts that the lobes of the mantle which cover the 

 shells of the cones and cowries are the respiratory organs of the animal 

 (Comp. Anat. i. 55) ; but this is a mistake ; they are truly pectini- 

 branchial. 



f Owen in Cyclop. Anat. and Phys. i. 542. See also Cuvier, Mem. i. 

 20. 



