| CEPHALOPODA. 
431 
suspecting that these belong to the Dibranchiate section 
of Cephalopods. 
The following table states the chief points of distinction 
between Mautilus and the other series of Cephalopods :— 
CEPHALOPODA 
TETRABRANCHIATA (Wautdlus). 
D1BRANCHIATA (Sepia, Octopus, etc.) 
Allextinct except one genus—Nautilus ; 
the extinct forms are usually ranked 
as Nautiloid and Ammonoid. 
Shell external, chambered, straight or 
bent or spirally coiled. That in which 
Nautilus lives has been described, 
with its siphuncle, gas-containing 
compartments, etc. 
The part of the foot surrounding the 
mouth bears a large number of lobes, 
which.carry tentacles in little sheaths, 
but no suckers. 
The two mid-lobes of the foot form a 
siphon, but they are not fused into a 
tube, ., 
The eye is without a lens, and is bathed 
internally by sea-water, which enters 
by a small pinhole aperture. There 
are two 
patches at the bases of the gills. 
phridia; two genital ducts (the left 
rudimentary). 4 
The ccelom sac -(pericardium) opens 
directly to the exterior by two aper- 
tures. 
The heart has two pairs of auricles, and 
there are no branchial hearts. 
No ink-bag. No salivary glands. 
“‘osphradia”” or smelling | 
“Two pairs of gills; two pairs of ne- 
;, Eledone moschata; in others an un- 
Numerous living genera, ranked as 
Decapods or Octopods; along with 
the former the extinct Belemnites are 
included. 
No living Dibranchiate lives in a shell. 
The shell was internal even in the 
extinct Belemnites, and in modern 
forms it occurs in various degrees of 
degeneration (cf. Spzrula, Sepia, 
Loligo), or is quite absent (Octopoda). 
The part of the foot surrounding the 
mouth is divided into ten or eight 
arms, which carry suckers, stalked in 
Decapods, sessile in Octopods. 
The two mid-lobes of the foot fuse to 
form a completely closed tubular 
siphon or funnel. 
The covering of the eye may be per- 
forated, but the mouth of the retinal 
cup is closed by alens, There are no 
osphradia, though there may be 
“olfactory pits” behind the eyes. 
One pair of gills; one pair of nephridial 
sacs; two oviducts in Octopoda and 
Oigopsida; two vasa deferentia in 
. paired genital duct. 
The ccelom opens into the nephridia 
by two pores, and thus to the ex- 
terior. 
The heart has two:auricles, and there 
are branchial hearts. 
Usually with an ink-bag. 
Salivary 
glands. 
CLASSIFICATION OF CEPHALOPODA. 
Order I. Tetrabranchiata (see Table). 
Family I. Nautilide. 
Nautilus alone alive; but a great 
series of fossil forms, Orthoceras— Trochoceras. 
Family II. Ammonitide. 
All extinct, but with shells well 
preserved, so that long series can be studied. They 
furnish striking evidence of progressive evolution in 
definite directions, e.g. Bactrztes, Ceratites, Baculites, 
Turrilites, Heteroceras, and the whole series of genera 
formerly classed as Ammonttes. 
