to say, in the extensions of its
area right and left between the various viscera--in different genera,
but in the Decapods is largest. In an extension of this chamber is
placed the ovary of _Sepia_, whilst the ventricle of the heart and the
branchial hearts and their appendages also lie in it. It is probable
that water is drawn into this chamber through the renal sacs, since
sand and other foreign matters are found in it. In all it opens into
the pair of renal sacs by an orifice on the wall of each, not far from
the external orifice (fig. 29, y, y'). There does not seem any room
for doubting that each orifice corresponds to the reno-pericardial
orifice which we have seen in the Gastropoda, and shall find again in
the Lamellibranchia.

[Illustration: FIG. 12.--Diagram to show the relations of the heart in
the Mollusca. (From Gegenbaur.)

A, Part of the dorsal vascular trunk and transverse trunks of a
worm.

B, Ventricle and auricles of _Nautilus_.

C, Of a Lamellibranch, of _Chiton_, or of _Loligo_.

D, Of _Octopus_.

E, Of a Gastropod.

a, Auricle.

v, Ventricle.

ac, Arteria=cephalica=(aorta).

ai, Arteria abdominalis. The arrows show the direction of the
blood-current.]

The circulatory organs, blood-vessels and blood of _Nautilus_ do not
differ greatly from those of Gastropoda. The ventricle of the heart is
a four-cornered body, receiving a dilated branchial efferent vessel
(auricle) at each corner (fig. 11). It gives off a cephalic aorta
anteriorly, and a smaller abdominal aorta posteriorly. The diagram,
fig. 12, serves to show how this simple form of heart is related to
the dorsal vessel of a worm or of an Arthropod, and how by a simple
flexure of the ventricle (D) and a subsequent suppression of one
auricle, following on the suppression of one branchia, one may obtain
the form of heart characteristic of the anisopleurous Gastropoda
(excepting the Aspidobranchia). The flexed condition of the heart is
seen in _Octopus_, and is to some extent approached by _Nautilus_, the
median vessels not presenting that perfect parallelism which is shown
in the figure (B). The most remarkable feature presented by the heart
of _Nautilus_ is the possession of four instead of two auricles, a
feature which is simply related to the metamerism of the branchiae. By
the left side of the heart of _Nautilus_, attached to it