SUB-CLASS II DIBRANCHIATA 593 
The body of Dibranchiates or Cuttle-fishes is elongated, cylindrical, or 
sack-shaped, and frequently provided with two lateral fin-like appendages. 
The anterior cephalic region gives off a circlet of eight or ten powerful, 
_ muscular arms, the inner sides of which are armed with suckers (acetabula), or 
a double row of hooks, and assist in swimming or creeping, and also serve for 
the capture of prey. The Sepioidea have two of 
their ten arms developed into very long tentacles 
which bear hooks or suckers only at their thickened 
extremities (Fig. 1236). The lower surface of 
the suckers is disk- or cup-shaped, perforated in 
the middle, and occupied by numerous radially 
arranged muscle fibres ; they are also occasionally 
furnished with horny hooks or sharp claws. Each 
sucker is able to create a partial vacuum by 
pressing the cartilaginous rim against some object 
and then contracting the inner folds, and hence 
can be used like a cupping-glass. 
The jaws resemble those of Nautiloids in 
form, but are never calcified ; owing to their 
perishable nature, usually horny, they are not 
preserved in the fossil state. The cartilage of 
the head forms a complete ring enclosing the 
central portion of the nervous system. The eyes 
are of a large size, protected by a capsule, and 
recall those of vertebrates in structure. 
The body is constricted at the mantle opening, 
which occurs just behind the head, and at this 
point on the ventral surface is placed the respira- 
tory orifice, bounded by a projecting fold of the 
mantle. Here also terminates the cylindrical or Fic. 1236. 
conical funnel, on either side of which lie the — Enoploteuthis leptura. Recent; 
: : : : Pacific Ocean. A, Ventral aspect. 
dendriform gills ; in the same neighbourhood too & tternal shell. 
are placed the anal and genital openings. 
The abdomen is sack-shaped, and contains besides the viscera and circu- 
latory systems a rather large pyriform vessel called the ink-bag. Its reservoir 
is filled with an extremely opaque brownish-black fluid, which can be voided 
at will through an excurrent canal terminating near the anus. Menaced or 
alarmed, the creature discharges an impenetrable cloud of ink, which serves to 
conceal its retreat. One often finds within the body of fossil Dibranchiates 
not only a cast or mould representing the ink-bag, but often a dark-coloured 
residuum of the carbonaceous particles suspended in the ink. 
The abdomen is completely covered by the mantle, which is a thick and 
frequently brilliantly coloured muscular envelope. ‘Traces of it are occasion- 
ally found among fossil forms, owing to a slight secretion of calcareous matter 
within it. 
Most Dibranchiates secrete an internal shell within the mantle. Only 
among the Octopoda is a shell absent entirely, or replaced in the female by a 
thin, simple, unchambered spiral; but this last is in nowise homologous with 
the usual Dibranchiate shell. Spirula has a spiral, camerated shell, the septa 
of which are traversed by a siphuncle, and whose coils are not in contact. It 
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