30 



It militates, however, against this idea of the spongy bodies of the veins of the 

 Cephalopods being a vestige of the structure just described, tliat in the present 

 instance they exist together. In Aplysia the orifices of communication are found 

 in the trunks of the vessels going to the branchiae after having received all 

 the other venous trunks of the body ; but in Nautilus this structure occurs only 

 in that part of the venous system which is in relation with the abdominal cavity, 

 and anterior to the reception of the great splanchnic veins ; hence it appears to 

 have special reference to the abdominal, or at least to a great serous cavity. In 

 both instances a peculiar muscular structure of the vein is demonstrable at the 

 part where these orifices occur, so that their function appears to depend on, or 

 to be in connexion with, a power of regulating their diameters. 



The vena cava, having arrived in the manner just described at the septum of 

 the pericardium, perforates that septum, and being joined by large trunks which 

 return the blood from the liver, ovary, gizzard, and remainder of the alimentary 

 canal, is dilated into a small membranous sinus of a transverse form (2. pi. 6.). 

 It is here, therefore, that the greater circulation terminates, if we are to consider 

 the lesser circulation to commence •<vhen the blood again begins to move from 

 trunks to branches : and, according to this view, the branchiae will be described 

 before the account of the vascular system is proceeded with. 



The respiratory organs of the Pearly Nautilus have a similar elongated pyra- 

 midal form, together with the same laminated structure and symmetrical dis- 

 position as in the Cuttle-fish ; but they are four in number, being disposed two 

 on either side, and each pair arising by a common peduncle from the inner sur- 

 face of the mantle. 



From this difference in the number of branchiae, in addition to the other 

 peculiarities in the structure of Nautilus, the existence of at least two orders 

 in the class Cephalopoda is, I imagine, demonstrated ; and the denominations 

 of these orders might conveniently be taken from the modifications of the 

 respiratory system. Assuming, therefore, that it is common to the class to 

 possess branchiae of a laminated structure, symmetrically disposed, and con- 

 cealed beneath the mantle, those genera which possess two such branchiae will 

 form an order under the term Dibranchiata, and the Pearly Nautilus and other 

 Cephalopods with shells of an analogous formation a second order, under the 

 term Tetrahranchiata. It is in this sense that the expression " Dibranchiate 



