informs me that he has been unable
to detect uric acid in it. The follicular appendages of the branchial
arteries present remarkable differences in their external appearance.
The eight which hang into the four anterior chambers are similar,
slightly festooned, but otherwise simple lamellae; while the four which
depend into the posterior chambers are produced into a number of
papillary processes. This external difference is obvious enough: whether
it be accompanied by a corresponding discrepancy in minute structure I
am unable to say; for I have not as yet been able to arrive at any
satisfactory results from the microscopic examination of the altered
tissues, and, as will be seen below, the only observer who has had the
opportunity of examining the Nautilus in the fresh state has not noted
any difference of structure in the two sets of follicles.

One is naturally led to seek among other mollusks for a structure
analogous to the vast posterior aquiferous chamber of the Nautilus; and
it appears to me that something quite similar is offered by the
_Ascidioida_ and the _Brachiopoda_. In both cases, the viscera, inclosed
within a delicate tissue, project into a large cavity communicating
freely with the exterior by the cloacal aperture in the one case, and
by the funnel-shaped channels which have been miscalled "hearts" in the
other.

The rudimentary renal organs of the Ascidian are developed in the walls
of the cavity in question; and an aquiferous chamber of smaller
dimensions has the same relation to the kidney in Lamellibranchiata--in
Gasteropoda, Heteropoda, Pteropoda, and dibranchiate Cephalopoda. But
although such is likely enough to be the case, we do not know at present
that the aquiferous chambers in any of the last named mollusks attain an
extension similar to that which obtains in Nautilus.

On comparing the observations detailed above with the statements of
previous writers, I find that, in his well-known "Memoir on the Pearly
Nautilus" (1832), Professor Owen describes "on each side, at the roots
of the branchiae," "a small mamillary eminence with a transverse slit
which conducts from the branchial cavity into the pericardium. There is,
moreover, a foramen at the lower part of the cavity (_o_, pl. 5)
permitting the escape of a small vessel; and by the side of this vessel
a free passage is continued between the gizzard and ovary into the
membranous tube or siphon that traverses the divisions of the shell,
thus