194 Dr. T. Williams on the Mechanism of Aquatic 



its continuation, the great anterior vena cava, appear to form 

 closed-ofF portions merely of the peritoneal cavity. The vena 

 cava in Nautilus is a flattened spacious channel, the parietes of 

 which are perforated by numerous orifices which communicate 

 directly with the peritoneal cavity. This remarkable peculiarity 

 is thus described by Professor Owen : "There are several small 

 intervals left between the muscular fibres and the corresponding 

 round apertures in the membrane of the vein (vena cava) and 

 contiguous peritoneum, by which the latter membrane becomes 

 continuous with the lining membrane of the vein ; from this 

 structure it would seem that the hlood might flow into the peri- 

 toneal cavity, or the fluid contents of that cavity be absorbed 

 into the vein*.'^ 



Though, from the small size of the specimens upon which the 

 author's observations have been conducted, he has not been 

 successful in his attempts to verify the statements of Prof. Owen, 

 he thinks it very probable, on the ground of analogy, that they 

 are true. In the Echinoderms and Annelids the vascular system 

 undoubtedly communicates with the peritoneal cavity. Of the 

 Gasteropod Mollusks Milne-Edwards observes, " L'artere aorte, 

 parvenue au point oil le canal digestif se recourbe pour descendre 

 de la face superieure du bulbe pharyngien dans la cavite abdo- 

 minale, debouche directement dans une vaste lacune, dont les parois 

 sont formees en partie par les teguments communs de la tete et 

 en partie par les muscles et les tuniques du pharynx jointes k 

 les lames de tissu connectif etendues transversalement au devant 

 de la cavite abdominalet-" The cephalic sinus in the Gastero- 

 pods forms a part of the arterial system ; in the Cephalopods it 

 is a segment of the venous system. That portion of the circu- 

 latory system of the Cephalopod which is intermediate, in the 

 solids of the body, between the arteries and veins, partakes much 

 more strikingly of the capillary character, or less of the lacunose, 

 than it does in any other Mollusk. It seems, from the researches 

 of Prof. Owen, that in Nautilus and Octopus, and other genera, 

 the pericardium (or that membrane which embraces the large cen- 

 tral vessels) opens immediately into the branchial chamber. Dr. 

 Lacaze Duthiers J has lately shown that the pericardial chamber in 

 theLamellibranchiata also opens externally. From these analogies 

 the writer of this paper believes that the lung-sac of the pulmo- 

 nary Gasteropods should be looked upon as the pericardial bag 

 slightly diverted from the character which is normal to it in 

 other Mollusks. 



Considered from this homological point of view, the lung of 



* Cvcloj). Anat. and Phys., Art. Cephalopoda, 

 t Aim. d. Sc. Nat. 3 ser. 1847. 

 X Ann. d. Sc. Nat. No. 5, 1855. 



