760 SIPHUNCLE AND PALLIAL VESSELS. 



above, of the automatic injection of the pallial veins with gas (PI. LXXXII.), it seems 

 that the conclusion may be drawn that the separation of gas rich in nitrogen is a special 

 property of the venous system of Nautilus. Whether this supposition will prove to be 

 sound or not, the observations are sufficiently singular to merit the attention of future 

 investigators. 



The sj3ong)' trabecular tissue of the siphuncle becomes looser, i.e. the meshes become 

 larger near the root of the siphuncle, and where the latter abuts upon the body-wall 

 there is generally to be seen from the inside of the perivisceral coelom a large cushion, 

 divided superficially into two equal or nearly equal portions, by the insertion of the 

 genito-intestinal ligament. In certain conditions of preservation the wall of this sac 

 is very easily ruptured, so as to produce the appearance of a natural orifice, although 

 such is not and cannot be present, since the cavity of the sac is the radical sinus of 

 the siphuncular vein, and opens directly into the posterior pallial vein. When the sac is 

 cut open it is found to contain a fiocculent substance, composed partly of blood, and 

 partly of the trabeculae described above. 



The radical sinus of the siphuncle of Nautilus is obviously identical in principle 

 with the pallio-siphonal sinus described by Huxley and Pelseneer' in their joint work 

 on Spirula. In Spirula we are told that the cavity of the membranous siphuncle is 

 a blood sinus which opens at the root of the siphuncle into a relatively large space, 

 which is the pallio-siphonal sinus. The irrefutable demonstration of the existence of 

 corresponding connections in Nautilus seems to me to be a matter of uncommon interest. 



The peculiar segmentation of the siphimcle due to shallow constrictions at the points 

 where it traverses the septa was first observed by Valenciennes. According to Appellof 

 {op. cit. 1893, p. 79 et seq.), it is open to question how far back the living siphuncle 

 extends in the adult shell, and it is even quite certain that it does not extend into 

 the fii-st chamber, since he has been able to confirm Hyatt's statement "dass auch der 



Sifo der zweiten Kammer wirklich blind geschlossen ist." In extracting the 



animal from the shell, vaiying lengths of the siphuncle emerge. I cannot assert that I 

 have ever obtained a complete siphuncle, but often one with upwards of twenty segments. 



I made some experimental observations on Nautili with severed siphuncle for the 

 purpose of ascertaining whether this organ performed a definite immediate function. The 

 result, so far as it went, seemed to indicate that the cutting of the siphuncle does not 

 temporarily affect the vitality of the animal, does not inhibit movements of translation 

 and does not prevent it from floating at the surface, nor from sinking to the bottom. 

 Its function must therefore be a cumulative one, a conclusion in accord with the 

 suggestion that it may serve to keep the air-chambers at the proper tension. I found 

 that the best way of performing the operation was to saw through the shell in the 

 neighbourhood of the posterior portion of the body of the animal over the cardiac region. 

 If the shell with the li\'ing animal inside be held mouth downwards, the cardiac region 

 lies approximately in the same vertical transverse plane with the points at which the 



' Huxley, Rt. Hon. T. H. , and Pelseneer, P., " Report on the specimen of the genus Spirula collected 

 by H.M.S. Challenger." Published as the 83rd and last part of the Zoological Series of Challenger Reports, 

 and inserted as an Appendix to the Summary of Results, second part, 1895. Plates by Huxley, text by 

 Pelseneer. For the account of the pallio-siphonal sinus see p. 24, text-fig. Q. 



