382 



Div. 2. MOLLUSCA.— CEPHALOPODA. 



and the Decapoda, or ten-armed Cuttle-fish. The Ocfopod tribe consists of the two families Testacea a,nd Nuda, ; 



to the former belongs the genus Argonauta, n-ith (perhaps) the fossil genus Bellerophon ; to tlie latter the genus 



Octopus, represented in the preceding figure (2) as creeping on 

 the shore with its mouth downwards. The Decapod tribe, which 

 includes the greater proportion of the existing Cephalopoda, is di- 

 vided into four families ; — the Teiithidoc or Calamaries, including the 

 genera Loligo, Sepiotcuthis, Oiiychoteuthis, Sepiola, Cra)\ch.ia, and XoJj- 

 gnpsis (Fig. 3) ; the Sepiadce, or common Cuttle-fish, comprised under 

 the single genus Sepiia ; the Spindidcc, consisting of the single genus 

 Spinila, which is a Decapodous Cephalopod, with an internal spiral 

 chambered shell, furnished with a siphon ; and the Belcmnitidce, a 

 group known only by fossil remains, but determined from these to 

 have been Decapodous Cephalopods, possessing conical chambered 

 shells of which a description has been given in the text. From cer- 

 tain markings on the surface of these shells, and from the fact that 

 distinct remains of an ink-bag have been frequently met with in the 

 last or largest chamber of the cone, it has been argued that, not- 

 withstanding the strong resemblance of the shell to that of many 

 genera allied to the true Nautilus and belonging to the Tetrahraii- 

 chiate group, the animal must have been Dibranchiate, and must have 

 included the shell, together with its massive sheath, in the same 

 manner as the Cuttle-fish includes the "pounce-bone." The Nau- 

 tilus possesses no ink-bag, its power of completely withdrawing the 

 body into its shell rendering such a means of protection unneces- 

 sary ; and the ink-bag seems to be wanting in the several fossil 

 Fig. 3.— LoLioorsis. genera, whose shells bear a strong resemblance to that of this genus. 



If, then, the ink-bag be peculiar to the Dibranchiate order, and its presence indicates the general organi- 



lation of that order, the Belemnite must have belonged to an animal more or less closely allied to the Sepui. 



The justice of this view has been made evident by the recent discovery of speci- 

 mens of Belemnite, in which the soft parts of the animal are so well preserved 



as to enable their form and general structure to be distinctly traced. From 



these it has been ascertained that the arms were furnished with hooks, as in the 



Onychoteuthis ; and tliat tlie body had a pair of small lateral fins, situated at 



about the middle of its length. From the weight of its dense internal shell, the 



Belemnite may be supposed to have commonly maintained a vertical position ; 



and, as its chambered portion was provided with a siphuncle analogous to that 



of Nautilus, the animal probably had the power of ascending and descending in 



the water with facility. It would rise swiftly and stealthily to fix its claws in 



the belly of a fish swimming at the surface above ; and then, perhaps, as swiftly 



dart down and drag its prey to the bottom and devour it. We cannot doubt 



that, like the hooked Calamaries of the present seas, the ancient Belemnites were 



the most formidable and predaceous of their class. [See Professor Owen's Me- 

 moir on the Belemnite in the Philosophical Transactions for 1844.] 

 Order II. — Tetrabranchiata. The Cephalopodes of this order are provided 



with a large external univalve shell, symmetrical in form, straight, or convoluted 



on a vertical plane, and divided by a series of partitions into numerous cham- 

 bers, of which the last-formed is the largest, and alone contains the body of the 



animal ; a dilatable and contractile tube or siphon is continued from the pos- 

 terior part of the animal through all the partitions and chambers of the shell ; 



but the attachment of the shell to the body is effected by means of two strong 



lateral muscles, which are inserted into the walls of the lastcliamber. The arms 



are very numerous, short, and hollow, each containing a long, slender, retractile 



tentacle ; they arc destitute of suckers. The head is provided with a large flat- 

 tened disk, which, besides acting as a defence to the orifice of the shell, serves 



also, in all probability, as an organ for creeping along the ground, like the foot 



in the Gasteropods. The jaws of the Tetrabranchiata are strengthened by a 



dense, exterior, calcareous coating, and have thick dentatcd margins. The eyes 



are pedunculated, and of a simple structure like those of the Gasteropoda. There 



is no organ of hearing. The gilXs are four in number, and vi-ithout brancliial 



hearts. The circulating system is provided with but one ventricle, which is 



systemic, or propels arterial blood. Tlicrc is no ink-bag. The inferior parictcs 



of the funnel are divided longitudinally. 



Of the Tetrabranchiate order, the only e.visting representative is the genus 

 NautUus, (Fig. fi,) whose general organization has been described in the previous account of the group. The 



Fio. 4. — Bri.kmnitk Restoredi 

 n, a, outer chniii})er of cone; b, cham- 

 bered portion ; c, spiithose guard ; tf, ink- 

 bag i t, r, mantle ; /, /, fins ; g, g, 

 hooked arms} i, funnel. 



