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 proostracum, 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_, Jurass