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As I could not detect any artery or vein, I conclude that they 
probably do not extend beyond the first chamber. This view 
coincides with Professor Owen’s statement * that “ neither the 
contents nor the vital properties of the siphon are, however, yet 
known ; an artery and vein are assigned for its life and nutri- 
tion, and to extend a low degree of the same influence to the 
surrounding shell : hut the structure of the membranous siphon, 
in the specimens from which I had the opportunity of examining 
it in a recent state, presents, beyond the first chamber, an in- 
extensible and almost friable texture, apparently unsusceptible 
of dilatation and contraction : it is also coated beyond the ex- 
tremity of the short testaceous siphon with a thin calcareous 
(nacreous) deposit.” 
We know that the body of the animal in Nautilus is attached 
to the shell by means of the two adductor muscles, and by a 
continuous horny girdle around the mouth of the body- 
chamber. The suggestion, therefore, of Von Buch, that the 
function of the siphuncle was to hold the animal into its shell, 
loses much of its significance. 
But may it not have been the most important point of at- 
tachment between the animal and its shell in the earlier for run 
of the Tetrabranchiata ? 
In support of this view we may notice that in the fossil 
Nautili it was a shelly tube of considerable size and thickness 
(Plate LXXXII. fig. 4), whilst in Orthoceras it attained to a 
great magnitude — as, for instance, in the genus Huronia , in 
which the siphuncle is as large as a human vertebral column. 
In Actinoceras , Gyroceras , and PJiragmoceras , the siphuncle is 
also very large, and contains in its centre a smaller tube, the 
space between the two being filled up with radiating plates, like 
the lamellse of a coral. 
Speaking of the connection between the Nautilus pompilius 
and its shell, Professor Owen f says : “ A third point of at- 
- tachment is to the bottom of the shell by the posterior ex- 
tremity of the mantle, which probably presents a conical form 
in the embryo Nautilus.” 
If, then, the siphuncle in the young stage forms the main 
point of attachment between the animal and its shell, we may 
reasonably argue that the siphuncle in the adult Nautilus is 
simply the evidence of an aborted embryonal organ whose 
function is now fulfilled by the shell-muscles, but which, in the 
more ancient and straight-shelled representatives of the group 
(the Orthoceratites), was not merely an embryonal but an im- 
portant organ in the adult. 
* “ Lectures on the Invertebrata,” XXIII., 2nd edition, 1855, p. 594. 
t Ibid. p. 592. 
