I. SEVIACEA. Although the animals of this division agree in many external and internal characters, there are, however, considerable differences of structure to warrant their subdivision into two orders. In the first order will be included such as are furnished with only eight arms, and whose sac is destitute of fin-like ex pansions, and either simple, or strengthened behind in the interior by two short corneous processes. The head is united to the sac behind, without the interven tion of a neck. The suckers have soft margins. The second will comprehend those which are furnished with eight arms and two feet, whose sac is furnished with fin-like expansions, and strengthened internally by cor neous or testaceous ribs or plates. The head is divided from the sac on all sides by a neck. The suckers have a corneous margin.
The first order might admit of some subordinate divisions, founded on differences in the habits of the animals. It will suffice that we notice merely the ge nera. These are three in number, Octopus, Eledona, and Ocythiie.
1. Octopus. This genus, so named by Lamarck, is the Polypus of Aristotle. All the arms are of equal size, and the suckers with which they are furnished are sessile, and arranged in a double row on their central aspect. The oviduct is double, and the margin of the anus is simple.
There are several species belonging to this genus ; but the one with which naturalists have been longest acquainted is the Sepia octopodia of Linnaeus, or Octopus vulgaris, of modern arrangements. There is a figure of it in Pennant's British Zoology, vol. iv. p. 53, No. 44. Tab. xxviii. The body is short, and rounded at the tail. The arms are nearly six times the length of the body, and furnished with about two hundred and forty suckers. This species inhabits the British seas, and has been termed Poor Cuttle, Pour-contrel, and Preke. It was first recorded as a native by \lerrat in his Pinare, p. It is occasionally dredged up from the oyster-beds in the Frith of Forth. Baker describes a species analo gous to this in the Philosophical Transactions for the year 1757, p. 777, Tab. xxix.
2. Eledona. This genus was known to Aristotle. It agrees with the preceding genus in having all the arms similar, but it is distinguished by each arm having only a single row of suckers.
Lamarck has figured and described two species of this genus in the Menz. de la Sac. d'Hist. Xot. One of these, a native of the Mediterranean, is remarkable for giving out an odour like musk.
3. Ocyth::;e. In this genus, so named by M. Rafi nesque, there are two of the arms furnished at their inner extremities with membranaceous expansions. The suckers, which are in a double row, are supported on short footstalks. Dr. Leach observed a four oblong spots on the inside of the tube, resembling surfaces for the secretion of mucus ; two inferior and lateral, and two supetior, larger, and meeting anteriorly. On the rim of the sac, immediately above the on each side, is a small, short, fleshy tubercle, which fits into an excavation on the opposite side of the sac. This character, which, with slight modifications, is com mon to this genus, to Loligo and Sepia, does not exist in the Polypus." Phil. Tr. vol. cvii. p. 295.
The animals of this genus were for a long time con sidered as the fabricators of the shell termed „eh-go:aura, or the Paper Nautilus, never having been found in any other condition than occupying the cavity of that shell. The clustered eggs (which they have in common with the other cephalopoda,) have likewise been found in the cavity at the top of the spire, the female occupying the mouth. They aro capable of raising themselves to the surface, and of floating and moving there by means of their winged feelers, aided probably by secreted air in their body. Upon the approach of danger they sink ra pidly to the bottom.
Although this animal has never been found but in the cavity of the shell of an Argonauta, there are various circumstances which confirm us in the belief, that it is not the fabricator of the shell, but, like the Hermit crab, occupies the deserted dwelling of another animal, which has not as yet fallen under the observation of naturalists. The body of the animal does not conform in shape to the cavity of the shell, nor to all its irregularities of surface. There is no muscular or ligamentous attachment be tween the animal and the shell, so that it is able to leave the shell at pleasure, as it was observed to do by the late Mr. Cranch, zoologist to the Congo expedition. The reader who is desirous of farther information on this subject, may consult Dr. Leach's Observations on the Genus OcythEie of Rafinesyue ; and Sir E. Home on the Distinguishing characters between the ova of the Sepia and those of the vermes testacea that live in water ; in the Philosophical Transactions for the year 1817, art. xxii. xxiii. ; and a paper by Mr. Say On the Genus Ocy tae in the Phil. Trans. 1 8 I 9, art. vii.