As our present observations are limited to the recent species of Cephalopoda, we pass over the Belemnites, which are fossil internal shells of extinct animals of this order, to speak of that of the Spirula. This is a small recent Cephalopod, respecting the precise form and organization of which nothing is yet satis factorily known. The only entire specimen which has been brought to Europe was taken by Peron, a French Naturalist, as it floated dead in the Tropical Ocean, between the Mol luccas and the Isle of France; it has been de scribed and figured by Roissy, Peron, and Lamarck ; but both the figures and descrip tions of these authors differ, and the specimen now no longer exists to determine the accuracy of either of the accounts. All agree, how ever, in stating that part of the shell was concealed within the body of the animal; and this fact is confirmed by a mutilated specimen in our own possession, and by one in a similar condition in the British Museum.
The shell of the Spirula (fig. 230) is about an inch in diameter, symmetrical, con voluted on one plane, with the whorls disjoined : it is composed of a succession of small regularly formed chambers, separated by partitions (a , a), which are concave towards the outlet of the shell, and are perforated by a siphon (b), the mem branous tube of which is protected by a series of funnel-shaped calcareous sheaths (c), which are continued from the hole of one septum into that of the next, throughout the shell. The shell is white, lined with a nacrous layer within, and partially covered by a straw-coloured epidermis without. The organization of the Spirula may be expected to be in some respects intermediate to the Nautilus and Sepia, and an opportunity of investigating its internal struc ture is therefore highly desirable. According to Lamarck the animal is a Cephalopod with eight feet and two tentacles, like a Cuttle-fish, all provided with suckers ; the body shaped like a purse and terminated behind by two lobes.
Although the siphoniferous shells are not confined to the Tetrabranchiate Order, yet it is in this division, as in the Pearly Nautilus for example, that we find this singular testaceous production to have arrived at the maximum of its development : it is covered by an epidermis, and, in the living animal, is also probably partially overlapped by a reflected portion of the thin and extensible mantle ; but no part of it is buried in the substance of the animal, whose entire body, on the contrary, is inclosed in the last large expanded chamber. The re
lative position of the soft parts to this cham ber we had not the means of determining from the specimen dissected by us, as this had been removed from its shell by Mr. Bennett, its fortunate captor, before it was placed in spirits. According to this able naturalist's statement, however, the ventral surface of the body and funnel was applied to the concavity of the outer expanded wall of the chamber; and the concavity behind the cephalic'disk was adapted to the involuted convexity of the shell, and abutted against the ridge which rises from that part.* The camerated portion of the shell, according to Mr. Bennett, contained water or a liquid ; but the size, condition, and con tents of the membranous tube were not ob served by him. The external form of the soft parts supported Mr. Bennett's account of their relative position to the shell; but some cir cumstances appeared to militate against the fluid nature of the contents of the deserted chambers. In the description of this spe cimen, we accordingly stated our belief that the chambers are naturally filled by a gaseous exhalation or secretion of the animal, and that the liquid is contained in the dilatable siphon which is extended from the posterior part of the animal's body, and passes through the central apertures of the different septa of the shell. From. the communication which this siphon has with the pericardial cavity, it can be influenced, as to the quantity of fluid which it contains, by the actions of the Nautilus itself. A pneumatic and hydraulic apparatus for effecting the rising and sinking of the shell and its in habitant is thus established, and Dr. Hooke's ingenious conjecture of the use of the camerated part of the shell is confirmed;* but the relative positions of the gas and water would, accord ing to the above opinion, be the reverse of what Parkinsont supposed them to be. The full development of the theory of chambered shells, considered as hydrostatic instruments is, how ever in abler hands than ours; and the reader will'be gratified to learn that it forms the sub ject of a portion of the forthcoming Bridge water Treatise by Dr. Buckland.