of as great importance as those derived from the gills, by which the
members of these two orders are separated from one another.

ORDER 1. TETRABRANCHIATA (= Schizosiphona, Tentaculifera).

_Characters_.--The inrolled lateral margins of the epipodia are not
fused, but form a siphon by apposition (fig. 4). The circumoral lobes of
the fore-foot carry numerous retractile tentacles, not suckers (fig. 6).
There are two pairs of ctenidial gills (hence Tetrabranchiata), and two
pairs of renal organs, consequently four renal apertures (fig. 4). The
viscero-pericardial chamber opens by two independent apertures to the
exterior, and not into the renal sacs. There are two oviducts (right and
left) in the female, and two sperm-ducts in the male, the left duct in
both sexes being rudimentary. A large external shell, either coiled or
straight, is present, and is not enclosed by reflections of the
mantle-skirt. The shell consists of a series of chambers, the
last-formed of which is occupied by the body of the animal, the hinder
ones (successively deserted) containing gas (fig. 1). The pair of
cephalic eyes are hollow chambers (fig. 14. A), opening to the exterior
by minute orifices (pin-hole camera), and devoid of refractive
structures. A pair of osphradia are present at the base of the gills
(fig. 4, _olf_). Salivary glands are wanting. An ink-sac is _not_
present. Branchial hearts are _not_ developed on the branchial afferent
vessels.

[Illustration: FIG. 1.--Lateral view of the female Pearly Nautilus,
contracted by spirit and lying in its shell, the right half of which is
cut away (from Gegenbaur, after Owen).

a, Visceral hump.

b, Portion of the free edge of the mantle-skirt reflected on to the
shell,--the edge of the mantle-skirt can be traced downwards and
forwards around the base of the mid-foot or siphon i.

l, l, Superficial origin of the retractor muscle of the mid-foot
(siphon), more or less firmly attached to the shell, of which a small
piece (s) is seen between the letters l, l.

s, (farther back) points to the siphuncular pedicle, which is broken
off short and not continued, as in the perfect state, through the
whole length of the siphuncle of the shell, also marked s and s'.

o, points to the right eye.

t, is placed near the extremities of the contracted tentacles of the
outer or annular lobe of the fore-foot--the jointed tentacles are seen
protruding a little from their long cyl