27 



is another distinct cavity (n. pi. 5.), of a square shape, whicli contains the heart 

 and principal vessels, with the glandular appendages connected therewith. 



This cavity is analogous to that which Cuvier has denominated, in Octopus 

 vulgaris, " la grande cavite veineuse" ; but it is not here, as in that animal, divided 

 by a membranous septum running along the mesial line. To prevent the con- 

 fusion of ideas hable to arise from the use of the term " venous cavity", I shall 

 hereafter, in treating of this receptacle, term it Pericardium. If it should be ob- 

 jected that a pericardium is a shut sac, and that this cavity, in addition to 

 its extent, communicates with the exterior of the animal through the medium 

 of the branchial ca^'ity, — it may be urged in reply, that the same external com- 

 munication exists in the pericardium of the Sturgeon, the Ray, and the Shark, 

 through the medimn of the abdominal cavity. 



The pericardium of the Pearly Nautilus is separated from the branchial cavity 

 by a strong membranous partition («'. u. pi. 5.), in which the following orifices are 

 observable : in the middle, the terniination of the rectum ; to the right of this, 

 the orifice of the oviduct ; and on each side at the roots of the branchiae there is 

 a small mammillary eminence, with a transverse sUt, which conducts from the 

 branchial cavity to the pericardium. There is, moreover, a foramen at the lower 

 part of the ca^'ity (o. pi. 5.), permitting the escape of a small vessel ; and by the 

 side of this vessel a free passage is continued between the gizzard and ovary 

 into the membranous tube or siphon that traverses the divisions of the shell ; thus 

 establishing a communication between the interior of that tube and the exterior 

 of the animal. 



The peripheral parietes of the pericardium are not distinct from the substance 

 of the mantle, but cohere, and form with it a thin but tough parchment-like 

 membrane ; having, in place of the thick fleshy sac of the Cuttle-fish, a suf- 

 ficient protection in the more extensively developed calcareous covering of the 

 Nautilus. The venous branches from the labial and digital tentacles and ad- 

 jacent parts of the head and mouth, terminate with those from the funnel in the 

 sinus excavated in the body of the cartilaginous skeleton. From this sinus the 

 great vena cava (1. pi. 6.) is continued, running in the interspace of the shell- 

 muscles on the ventral aspect of the abdominal cavity, and terminating in a 

 sUghtly dilated part (2. pi. 6.) just within the pericardium, where it receives, by 

 two large trunks (3. 3. pi. 6.), the veins of the different viscera. The structure of 

 the vena cava is very remarkable. It is of a flattened form, being included be- 



E 2 



