shut off from this general blood-lymph system, and communicates,
directly in _Nautilus_, in the rest through the renal sacs, with the
exterior. In the Cephalopoda this specialized pericardial cavity is
particularly large, and has been recognized as distinct from the
blood-carrying spaces, even by anatomists who have not considered the
pericardial space of other Mollusca to be thus isolated. The enlarged
pericardium, which may even take the form of a pair of sacs, has been
variously named, but is best known as the viscero-pericardial sac or
chamber. In _Nautilus_ this sac occupies the whole of the
postero-dorsal surface and a part of the antero-dorsal (see fig. 10,
x), investing the genital and other viscera which lie below it, and
having the ventricle of the heart suspended in it. Certain membranes
forming incomplete septa, and a curious muscular band--the
pallio-cardiac band--traverse the sac. The four branchial afferent
veins, which in traversing the walls of the four renal sacs give off,
as it were, glandular diverticula into those sacs, also give off at
the same points four much larger glandular masses, which hang freely
into the viscero-pericardial chamber (fig. 11, r.e). In _Nautilus_ the
viscero-pericardial sac opens to the exterior directly by a pair of
apertures, one placed close to the right and one close to the left
posterior renal aperture (fig. 5, visc.per). This direct opening of
the pericardial sac to the exterior is an exception to what occurs in
all other Mollusca. In all other Molluscs the pericardial sac opens
into the renal organs, and through them or the one renal organ to the
exterior. In _Nautilus_ there is no opening from the
viscero-pericardial sac into the renal sacs. Therefore the external
pore of the viscero-pericardial sac may possibly be regarded as a
shifting of the reno-pericardial orifice from the actual wall of the
renal sac to a position alongside of its orifice. Parallel cases of
such shifting are seen in the varying position of the orifice of the
ink-bag in Dibranchiata, and in the orifice of the genital ducts of
Mollusca, which in some few cases (e.g. _Spondylus_) open into the
renal organs, whilst in other cases they open close by the side of the
renal organs on the surface of the body. The viscero-pericardial sac
of the Dibranchs is very large also, and extends into the dorsal
region. It varies in shape--that is