CEPHALOPODA. 



547 



are concave towards the outlet of the shell, 

 and are perforated by a siphon (6), 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 

 SpiruLi 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 tlie 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 pericardia! cavity, it can be 

 influenced, as to the quantity of fluid which it 



* M. De Blainville, in a learned Memoir on the 

 Structure of the Shells of Spirula and Nautilus, 

 states his opinion that the true position of the ani- 

 mal of the latter shell is the reverse of that de- 

 scribed above : this opinion has been adopted by 

 some Naturalists of this country, but the analogies 

 by which it is endeavoured to be supported are too 

 remote and vague to enforce conviction. 



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 

 Parkinson-j- 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. 



NERVOUS SYSTEM. In tracing the develop- 

 ment of the Nervous System through the 

 Heterogangliate or Molluscous type of Orga- 

 nization, we find in the Gasteropodous genera 

 which approach nearest to the Cephalopodous 

 or highest division, that the ganglions which 

 are concentrated about the head, are arranged 

 in three groups : one, which is supracesopha- 

 geal, supplies the sentient organs, as the eyes 

 and feelers ; a second, which is subresophageal 

 and anterior, supplies the buccal apparatus ; 

 a third, which is suboesophageal and pos- 

 terior, is the centre from which the sensitive, 

 motive, and plastic nerves of the trunk ori- 

 ginate. The anterior or buccal ganglions are 

 united together, and to the cerebral ganglions, 

 forming a nervous collar around the oesophagus; 

 a similar collar is formed by the corresponding 

 intercommunicating chords of the posterior 

 suboasophageal ganglia. 



In the Cephalopods the nervous system is 

 disposed on the same general plan, but the 

 nervous substance is accumulated in a greater 

 degree at the different centres of radiation, 

 according to the superior development of the 

 parts that are to be supplied therefrom. 



In the Tetrabranchiate Order the principal 

 parts superadded to the structure which we 

 observe in the Gasteropodous Mollusk are those 

 locomotive and prehensile organs which sur- 

 round the buccal apparatus ; and the chief 

 modification of the nervous system is therefore 

 seen in the enlargement of the oral ganglia 

 and collar, and their close approximation to 

 the cerebral ganglion. This part is compara- 

 tively little advanced, since the organs of 

 sense which it immediately supplies, retain 

 the same simple structure as in the inferior 

 class of Mollusks, and are only augmented 

 in bulk. The brain therefore is represented 

 by a thick round tranversely extended chord 

 (l,^zg. 231), communicating at its extremi- 

 ties with the anterior and posterior cesopha- 

 geal collars (3, 4), and with the small 

 optic ganglions (2, 2), which supply the sim- 

 ple pedunculated eyes. Four small pairs of 

 nerves (5) also pass from the supracesophageal 

 band to the fleshy mass supporting the man- 

 dibles. The cranial cartilage seems in the 

 Nautilus to be principally developed with re- 

 ference to the strong muscular masses to which 



* Philosophical Experiments and Observations, 

 p. 307. 



t Organic Remains, vol. iii. p 102. 



