MUSEUM OF COMPARATIVE ZOOLOGY". 93 



SIPHON. 



The siphon has been so much studied and so fully described that at 

 first sight it seems impossible to add anything to our knowledge in this 

 direction. The structure, however, in many of the most essential fea- 

 tures has been either misinterpreted, or only partially understood, on 

 account of the disuse of the microscope in palasontologieal researches. 

 The siphon in Nautilus Pompilius * begins with a closed coecal prolon- 

 gation of the first septum. This is circular in outline, apparently flat- 

 tened at the bottom, and rests directly upon the lower end of the cicatrix. 

 The ventral side is somewhat inclined, the dorsal more or less abrupt, 

 the ccecum swelling out below the septum in the specimen examined. 

 Above the first septum the siphon expands, becoming much larger. 

 The walls, as well as the bottom of the ccecum, are lined internally by 

 an extension of the sheath, which is continuous with the siphonal funnel 

 of the second septum. There is also in the figure on Plate IV a layer 

 lying between the internal corneous layer of the siphon and the sheath. 

 The walls of the third septum are continuous beyond the mouth of the 

 siphonal funnel of the second septum, and seem also to coat the inside 

 of the ccecum, but I have been unable to verify this observation upon 

 other specimens, and prefer to consider it doubtful. 



The structure of the outer wall, or sheath of the siphon, differs from 

 that of the septum itself, in being of a looser and more granular texture. 

 This, in the specimen examined, was more marked upon the dorsal than 

 upon the ventral side, between the apex and the first septum, as well as 

 between the latter and the second. At the third septum, however, this 

 difference is not so noticeable ; the denser portion of this septum, or the 

 siphonal funnel, extends posteriorly nearly as far on the dorsal as on 

 the ventral side. As previously described, the siphonal funnel rests 

 upon the looser-textured wall, and is really continuous with it. The 

 difference in texture merely results from the quicker formation of the 

 latter, while the animal is passing from one septum to the next in a«;e, 

 and the quieter deposition of the former while the animal is taking its 

 periodic rest, and building up a septum. 



The siphonal funnel is always spoken of as belonging to the septum, 

 and not to the siphon proper. This is true of the adult, but not of the 



* Plate IV, Fig. 4, R. 



