Letters in (3) to (9):--a, lateral fins of the mantle; b,
mantle-skirt; c, supra-ocular invagination to form the "white body";
d, the eye; e, the mouth; f^1, f^2, f^3, f^4, f^5, the five paired
processes of the fore-foot; g, rhythmically contractile area of the
yolk-sac, which is itself a hernia-like protrusion of the median
portion of the fore-foot; h, dotted line showing internal area
occupied by yolk (food-material of the egg); k, first rudiment of
the epipodia (paired ridges which unite to form the siphon or
funnel); l, sac of the radula or lingual ribbon; m, stomach; n,
rudiments of the gills (paired ctenidia); o, the otocysts--a pair of
invaginations of the surface of the epipodia; p, the optic ganglion;
q, the distal portion of the ridges which form the siphon, k being
the basal portion of the same structure; r, the vesicle-like
rudiment of the intestine formed independently of the parts
connected with the mouth, s, k, m, and without invagination; s,
rudiment of the salivary glands; t in (7), the shell-sac at an
earlier stage open (see fig. 36), now closed up; u, the open
shell-sac formed by an uprising ring-like growth of the
centro-dorsal area; w in (5), the mantle-skirt commencing to be
raised up around the area of the shell-sac. In (7) mes points to the
middle cell-layer of the embryo, ep to the outer layer, and h to the
deep layer of fusiform cells which separates everywhere the embryo
from the yolk or food-material lying within it.]
The coelomic cavity appears as a symmetrical pair of spaces in the
mesoderm, right and left of the intestine, and from it grow out the
genital ducts and the renal organs. The gonad develops from the wall
of the coelom.
[Illustration: FIG. 36.--Section through aboral end of embryo of
_Loligo_ showing shell-sac still open. ep, ectoderm; m, mesoderm;
m', endoderm; shs, shell-sac; y, yolk.]
_Phylogeny and Classification._--The order is divided into two
sub-orders, Decapoda and Octopoda, by the presence or absence of the
tentacular arms. The Decapoda are more adapted for swimming than the
Octopoda, the body being usually provided with fins. In the former
also there is generally an internal shell of considerable size, often
calcined, while in the Octopoda only the merest vestiges of a shell
remain. There can be no doubt that the Octopoda were derived from the
Decapoda, although from the absence of skeletal structures fossil
remains of Octopods are almost entirely unknown. _Palaeoctopus_,
however, occurs in the Cretaceous, while shells of _Argonauta_ do not
appear before the Pliocene. The Decapoda are abundantly represented in
the Secondary formations by the _Belemnitidae_, whose shell (fig. 19)
consists of a straight conical phragmacone covered posteriorly by a
very thick rostrum, and produced anteriorly into a thin long
proöstracum which is only occasionally preserved. In certain cases
remains of the arms provided with hooks, and of the ink-sac, have been
recognized. The _Belemnitidae_ appear first in the Upper Trias, attain
their maximum development in the Jurassic rocks, and are not continued
into the Tertiary period, though represented in the Eocene by a few
allied forms.
There is no difficulty in deriving the typical existing Decapoda from
_Belemnitidae_, and many of the extinct forms may have been directly
ancestral. Chitinous "pens" like that of _Loligo_, however, begin to
appear in the Jurassic and Cretaceous rocks, so that in this case as
in many others the parent form and the modified form existed
contemporaneously, and the latter alone has survived. The oldest
shells of the _Sepia_ type are from the Eocene, and it is perhaps
possible that the _Sepiidae_ arose separately from the Belemnites.
It is a curious fact that no fossil specimens of the genus _Spirula_
have been found, but this may be due to the fact that it occurs only
in deep water. At any rate there is no evidence that the shell of
_Spirula_ has lost a rostrum and a proöstracum; its characters must be
regarded as primitive, not secondary. In the characters of the
protoconch and of the commencement of the siphuncle, the shell of
_Spirula_ agrees with that of the Ammonoids, and in both its position
is ventral, although in most Ammonoids the shell being exogastric the
ventral side is the convex or external, while in _Spirula_ the shell
is endogastric and the siphuncle internal. The fact that the shell is
not completely enclosed by the mantle is also a primitive character.
With regard to the general morphology of the Cephalopoda, it is
difficult to reconcile the existence of two pairs of renal tubes as
well as a pair of genital ducts in _Nautilus_ with the view that the
original Mollusc was unsegmented and had only one pair of
coelomoducts. Considering the great specialization, however, and high
degree of organization of the Cephalopods, it is evident that the
earliest Nautiloid whose remains are known to us must have had a long
evolutionary history behind it, and such metamerism as exists may have
been developed in the course of its own history. In the other
direction the evidence seems to prove that the Dibranchiata with only
two renal ducts have been derived from the Tetrabranchiata.
SUBORDER 1. DECAPODA.--Four pairs of ordinary non-retractile arms
which are shorter than the body, and one pair of tentacular arms,
situated between the third and fourth normal arms on each side and
retractile within special pouches. Suckers pedunculated and provided
with horny rings, on the tentacular arms confined usually to the
distal extremities. Usually a well-developed internal shell, and
lateral fins on the edges of the body. Heart in a coelomic cavity;
nidamentary glands usually present.
[Illustration: FIG. 37.--Right and left sections through embryos of
_Loligo_. (After Lankester.)
A, Same stage as fig. 35 (4).
B, Same stage as fig. 35 (8); only the left side of the sections is
drawn, and the food-material which occupies the space internal to
the membrane ym is omitted.
al, Rectum.
is, Ink-sac.
ep, Outer cell-layer.
mes, Middle cell-layer.
ym, Deep cell-layer of fusiform cells (yolk-membrane).
ng, Optic nerve-ganglion.
ot, Otocyst.
wb, The "white body" of the adult ocular capsule forming as an
invagination of the outer cell-layer.
mtf, Mantle-skirt.
g, Gill.
ps, Pen-sac or shell-sac, now closed.
dg, Dorsal groove.
poc. Primitive optic vesicle, now closed (see fig. 34).
l, Lens.
r, Retina.
soc, Second or anterior optic chamber still open.
if, Iridean folds.
C, The primitive invagination to form one of the otocysts, as seen
in fig. 35 (5) and (6).]
Tribe 1. _Oigopsida_.--A wide aperture in the cornea. Two oviducts in
the female. In fossil genera and _Spirula_, shell has a multilocular
phragmacone with a siphuncle; initial chamber globular and larger than
the second chamber. The most ancient forms characterized by the small
size of the rostrum and proöstracum, and large size of the
phragmacone. In the living genera, except _Spirula_, the shell is a
chitinous gladius.
Fam. 1. _Belemnoteuthidae_. Extinct; shell with well-developed
phragmacone, and rostrum merely a calcareous envelope; siphuncular
necks directed backwards as in Nautiloidea; ten equal arms provided
with hooks. _Phragmoteuthis_, Trias. _Belemnoteuthis_, Jurassic and
Cretaceous. _Acanthoteuthis_, Jurassic.
Fam. 2. _Aulacoceratidae_. Extinct; phragmacone with widely
separated septa; rostrum well developed and claviform.
_Aulacoceras_, Trias. _Atractites_, Trias and Jurassic.
_Xiphoteuthis_, Lias.
Fam. 3. _Belemnitidae_. Extinct; phragmacone short with ventral
siphuncle, prolonged dorsally into long proöstracum; rostrum large
and cylindrical. _Belemnites_, 350 species from Jurassic and
Cretaceous. _Diploconus_, Upper Jurassic.
Fam. 4. _Belopteridae_. Extinct; rostrum and phragmacone well
developed, phragmacone often curved; initial chamber small.
_Beloptera_, Eocene. _Bayanoteuthis_, Eocene. _Spirulirostra_,
Miocene.
Fam. 5. _Spirulidae_. Dorsal and ventral sides of posterior
extremity of shell uncovered by mantle; no rostrum or proöstracum;
shell calcareous, coiled endogastrically and sipnunculated; fins
posterior. _Spirula_, three living species known, abyssal.
Fam. 6. _Ommatostrephidae_. Shell internal and chitinous, ending
aborally in a little narrow cone; tentacular arms short and thick;
suckers with denticulate rings. _Ommatostrephes_, fins aboral,
simple and rhomboidal, British. _Ctenopteryx_, fins pectinate, as
long as the body; _Bathyteuthis_, fins terminal, rudimentary;
tentacular arms, filiform; abyssal. _Rhynchoteuthis_, tentacular
arms united to form a beak-shaped appendage. _Symplectoteuthis.
Tracheloteuthis. Doridicus. Architeuthis_; this is the largest of
Cephalopoda, reaching 60 ft. in length including arms.
Fam. 7. _Thysanoteuthidae_. Arms enlarged, bearing two rows of
suckers and filaments; fins triangular, extending whole length of
body. _Thysanoteuthis_, Mediterranean.
Fam. 8. _Onychoteuthidae_. Fins terminal; tentacular arms long;
suckers with hooks. _Onychoteuthis_, hook-bearing suckers on
tentacular arms only. _Enoploteuthis_, hook-bearing suckers on all
the arms. _Veranya_, body very short, tentacular arms atrophied in
the adult, Mediterranean. _Chaunoteuthis_, body elongated,
tentacular arms atrophied. _Pterygioteuthis. Ancistroteuthis.
Abralia. Teleoteuthis. Lepidoteuthis._
Fam. 9. _Gonatidae_. Body elongated; fins terminal; radula with only
two lateral teeth. _Gonatus_.
Fam. 10. _Cheiroteuthidae_. Tentacular arms long, not retractile;
resisting apparatus well developed. _Cheiroteuthis_, suckers along
the whole length of the tentacular arms. _Doratopsis_, body very
long and slender with aboral spine, dorsal arms very short.
_Histioteuthis_, six dorsal arms united by membrane, photogenous
organs present. _Histiopsis_, membrane of dorsal arms only half-way
up the arms, photogenous organs present. _Calliteuthis_, no brachial
membrane, photogenous organs present. _Grimalditeuthis_, two fins on
each bide, no tentacular arms.
Fam. 11. _Cranchiidae_. Eight normal arms, very short; eyes
prominent; fins small and terminal. _Cranchia_, body short,
purse-shaped, normal arms short, fins entirely aboral. _Loligopsis_,
body elongated, conical, tentacular arms slender. _Leachia_,
tentacular arms absent, funnel without a valve. _Taonius_, body
elongated, normal arms, rather short, eyes pedunculated.
[Illustration: FIG. 38.--Octopodous Cephalopods.
A, _Pinnoctopus cordiformis_, Quoy and Gain (from New Zealand).
B, _Tremoctopus violaceus_, Ver. (from the Mediterranean).
C, _Cranchia scabra_, Owen (from the Atlantic Ocean; one of the
Decapoda).