my of _Nautilus
umbilicatus_, published in the Philosophical Transactions for 1855, thus
describes the follicular appendages of the branchial arteries:--

"These follicles are subcylindrical in form, somewhat dilated at the
free extremity, to which is appended a folded and funnel-shaped process
of membrane, which expands rather suddenly, presenting a jagged and
irregular border. They open by a smooth and oval or slit-like, orifice
into the afferent pulmonary vessels, on each of which, as Professor Owen
has observed, they are disposed in three clusters. The outer membrane is
smooth and glassy, homogeneous in structure and sprinkled over with
minute rounded and transparent bodies, probably the nuclei of cells.
Beneath this layer, flat bundles of fibres, apparently muscular, are
traceable here and there, principally disposed in a longitudinal
direction, and sometimes branched. The lining membrane consists of a
loose epithelial pavement in many respects similar to that of the
uriniferous tubules of the higher animals, the cells containing, besides
the nuclei, numerous minute oil-globules, or a substance much resembling
concrete fatty matter. This membrane is thrown up into an infinite
number of papillae and corrugations, so as to augment the extent of
surface considerably. The papillae are more numerous at the inner part or
towards the attached end; and a circlet of longitudinally disposed folds
radiate from the bottom of the follicles, in which a number of small
pits or fenestrations are sometimes visible. The sides of these folds
are wrinkled transversely so as to present a median zigzag elevation.
The funnel-shaped membranous process above noticed is continuous with
the lining membrane, consisting of an extension of the same epithelial
pavement; but the cells are somewhat larger and more regular in form.
The cavity of each follicle, therefore, communicates with the exterior
through the centre of this process; and the aperture is thus guarded by
a kind of circular valve, permitting the escape of secreted matter, but
effectually preventing the entrance of fluid from without."

In his fig. 9, pl. xv., Mr. Macdonald depicts certain "crystalline
bodies often occurring within the follicles."

From what Mr. Macdonald states, one would be led to conclude that all
the follicles have the same structure; but I suspect this to be an
oversight.

[Illustration: _Nautilus pompilius._ Fig. 1.

Viewed from the left side and a little