sion 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_,