ON THE STRUCTURE OF CAMERATED SHELLS. 
117 
“ From the extremity of the sac is continued a small tubular 
membranous process, which passes through the siphonic apertures 
in the septa of the shell, and is continued, there is reason to 
believe, to the innermost chamber. This tube has been surmised 
to be tendinous or muscular ; but the attachment of the shell 
to the soft parts proves to be effected by much more adequate 
means. Eumphius appears to have been acquainted with its 
true structure, for he calls it an artery (een langen ader ), and 
in fact within the external thin membrane are included a small 
artery and vein. How far these vessels are continued within 
the chambered portion of the shell, or in what manner they are 
distributed, remains for some future investigation ; for in the 
present instance the only part of the shell that was preserved 
was the small portion adhering to one of the horny tendons, and 
the membranous tube had been ruptured, in removing the 
animal, at a few lines distance from its origin at the mantle. 
This tube appears to be contracted at its origin, and its diameter 
at the wider part is one line and a half.” * 
Admitting, however, that the purpose of the siphuncle is to 
maintain the vitality of the shell during the long life of the animal, 
how, we venture to ask, can this vitality be maintained in a 
non-vascular body ? If the siphuncle be a means for repairing 
the shell, we ought to find some connection between it and the 
shell, but such does not exist ; indeed, the fossil species have, in 
many instances, enormously thick nacreous or shelly tubes. 
In fact, when the shell of the Nautilus, or of any other 
Mollusk, is once formed, it is extravascular, or dead matter , in 
the same sense that nails and hoofs and hair of higher animals 
are so — being incapable of repair, save at the growing end or 
where in contact with the shell-secreting mantle. 
In the specimen of Nautilus umbilicatus already referred 
to, which I had the good fortune to examine, I observed the 
thin pellicle of membrane, described by Professor Owen, lining 
the chambers ; but as it is only a film and presents no structure 
under the microscope, I conclude it to be deposited or left 
behind by the secreting surface of the mantle when the nacreous 
septum was formed. And this opinion is strengthened by Dr. 
Carpenter’s statement 66 that in every distinct formation of shell- 
substance there is a single layer of membrane,” and 66 that this 
membrane was at one time a constituent part of the mantle of 
the mollusc-” f 
The nacreous covering of the siphuncle was entire, and on 
removal it was found to enclose a simple membranous tube, 
composed of an extension of the periostracum , and exhibiting 
no structure even under a one-tenth objective. 
* Professor Owen’s u Memoir on the Pearly Nautilus,” 1 p. 10. 
f 11 British Association Reports>” 1844, p. 9. 
