The anterior ventral[398] portion of the mantle is furnished with a singular contrivance for locking it to the funnel, and so rendering the whole animal more capable of resisting the impact of any force. This contrivance generally consists of a series of ridges or buttons which fit into grooves or button-holes, the ridges being on the interior face of the mantle and the corresponding grooves on the funnel, or vice versâ. The ‘resisting apparatus’ is most elaborate in the pelagic genera, and least so in the more sluggish littoral forms. A similar, but not so complex, arrangement occurs also in the Octopoda.
The different forms of the shell appear to indicate successive stages in a regular course of development. We have in Spirula (Fig. [247]) a chambered shell of the Tetrabranchiate type, but of considerably diminished size, which has ceased to contain the animal in its last chamber, and has become almost entirely enveloped in reflected folds of the mantle. These folds gradually concresce to form a definite shell-sac, by the walls of which are secreted additional laminae of calcareous shell-substance. These laminae invest the original shell, which gradually (Spirulirostra, Belosepia) loses the spiral form and becomes straight, eventually disappearing, while the calcareous laminae alone remain (Sepia). These in their turn disappear, leaving only the plate or ‘pen’ upon which they were deposited (Loligo), which itself also, with the shell-sac, finally disappears, surviving only in the early stages of Octopus (Lankester).
The Decapoda are divided, according to the character of the shell, into Phragmophora, Sepiophora, and Chondrophora.[399]
Fig. 245.—‘Club’ of Onychoteuthis sp., showing the hooks and clusters of fixing cushions and acetabula below them, × ½.
A. Phragmophora.—Arms furnished with hooks or acetabula; shell consisting of a phragmocone or chambered sac enclosed in a thin wall (the conotheca), septa pierced by a siphuncle near the ventral margin (in Spirula alone this chambered sac forms the whole of the shell). The apex of the cone lies towards the posterior end of the body, and is usually enveloped in a calcareous guard or rostrum. Beyond the anterior end of the rostrum the conotheca is extended forward dorsally by a pro-ostracum or anterior shell, which may be shelly or horny, and corresponds to the gladius of the Chondrophora. The rostrum consists of calcareous fibres arranged perpendicularly to the planes of the laminae of growth, and radiating from an axis, the so-called apical line, which extends from the extremity of the phragmocone to that of the rostrum. Distribution, see p. [380].
Fam. 1. Spirulidae.—Arms with acetabula, shell a loose spiral, without rostrum or pro-ostracum, partially external, enclosed in two lobes of the mantle (Figs. [247] and [248]).
The single species of the single genus (S. Peronii Lam. = laevis Gray) has not yet been thoroughly investigated, although the shell occurs in thousands on many tropical beaches, and is sometimes drifted on our own shores. The animal appears to have the power of adhesion to the rocks by means of a terminal sucker or pore. The protoconch is present, and contains a prosiphon, which does not connect with the siphuncle. The septal necks are continuous, not broken as in Nautilus. The siphuncle is on the ventral margin of the shell, the last whorl of which projects slightly on the dorsal and ventral sides, but is even there covered by a thin fold of the mantle. The retractor muscles of the funnel and of the head find their point d’appui on the shell, the last chamber of which contains the posterior part of the liver, with which the membranous siphuncle is connected.
Fam. 2. Belemnitidae.—Arms hooked as in Onychoteuthis, fins large; phragmocone straight, initial chamber globular, larger than the second, rostrum often very long, investing the phragmocone, pro-ostracum sword- or leaf-shaped, rounded in front, seldom preserved, ink-sac present.—Lower Lias to Cretaceous.
