M O L L U S C A. 



609 



Molluict- 1. SCPIACEA. Although the animals of this division 

 ' , 'i ' agree in many external and internal characters, there 



Spiacca. aTC( 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 expansion*, 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 

 genera. These are three in number, Octopus, Eledona, 

 and OcythiJe. 



5ctopj. 1. Oclopus. This genus, so named by Lamarck, is 

 the Polypus of Aristotle. All the arms are of equal 

 size, and the sucker* with which they are furnished 

 are tetsile, and arranged in a double row on their cen- 

 tral aspect. The oviduct u double, and the margin of 

 the anus is simple. 



There are several specie* belonging to this genus ; 

 but the one with which naturalists nave been longest 

 acquainted is the Sepia octopodia of Linnaeus, or Octopus 

 vuigarit, of modern arrangements. There is a figure of 

 it in I'ennant's British Zoology, vol. iv. p. .33, 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 

 ha* been termed Poor Cuttle, Pour-contrel and Prekc. 

 It was first recorded as a native by Mcrrat in his Pin- 

 are, p. 191. It i* occasionally dredged up from the 

 oyster beds in the Frith of Forth. Baker describe* a 

 specie* analogous to this in the Philosophical Traiuac- 

 ttont for the year 1757, p. 777, Tab. MIX. 



[(dona. 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 on- 

 ly a single row of sucker*. 



Lanark ha* figured and described two species of 

 this genus in the Mem. de la Sue. fllist. Nat. One of 

 these, a native of the Mediterranean, is remarkable for 

 giving out an odour like musk. 



fc)U9. 9. Ocylh'Je. In this genus, so named by M. Rafi- 

 nesque, there are two of the arms furnished at their 

 inner extremities with membranaceous expansion*. 

 The suckers, which are in a double row, are supported 

 on short footstalk*. Dr. Leach observed " four oblong 

 spots on the inside of the tube, resembling surfaces for 

 the secretion of mucus ; two infciior and lateral, and 

 two superior, larger, and meeting anteriorly. On the 

 rim of the sac, immediately above the branchiae, on 

 each side, i* 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. I 



The animals of this genus were for a long time con- 

 sidered ai the fabricators of the shell termed Argonaula 

 or the Paper Nautilus, never having been found in any 

 other condition than occupying the cavity of that shell. 

 The clunered eggs (which they have in common with 



OL. XIV. PART II. 



the other cephalopoda,) have likewise been found in the X'ollu sc. 

 cavity at the top of the spire, the female occupying the '"Y 

 mouth. They are 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 

 rapidly to the bottom. 



Although this animal has never been found but in 

 the cavity of the shell of an Argonauta, there are vari- 

 ous 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 na- 

 turalists. The body of the animal does not conform in 

 shape to the cavity of the shell, nor to all its irregula- 

 rities of surface. There is no muscular or ligamentous 

 attachment between 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 in- 

 formation on this subject may consult Dr. Leach's Ob- 

 servations on the Genus Ocylhoe oj Rqfinesque ; and Sir 

 E. Home on the Distinguishing character* between lh 

 ova of the Sepia and those of the venues testacea thai 

 live in mater ; in the Philosophical Transactions JOT the 

 year 1817, art. xxii. xxiii. ; and a paper by Mr. Say 

 'On I he Genus Ocythut in the Phil. Trans. 1819, art. vii. 



The genera which belong to the second order are on- 

 ly two in number, viz. Sepia and Loligo. In both, the 

 suckers on the arms are pedunculated, and their margin 

 strengthened by a corneous ring furnished with teeth, 

 and the margin of the anus is surrounded with ap- 

 pendages or tentacula. The two feet with which the 

 animals of this order arc furnished are nearly similar in 

 their structure to the arms, but considerably larger in 

 their dimensions) They take their rise on the ventral 

 side of the mouth, between that organ and the funnel. 



4. Sepia. In this restricted genus the sac is furnish- Sepia. 

 ed on each side throughout its whole length with a nar- 

 row fin, and strengthened on the back by a complicated 

 calcareous plate, lodged in a peculiar cavity. The 

 suckers are irregularly scattered on the arms and feet. 



'1 he calcareous plate has been long known under the 

 name of cttttle-Jish bone. It is somewhat ovate, flatly 

 convex on both sides, and thickest where broadest. 

 The superior half, or the one next the head, is the long- 

 est, rounded at the extremity, and thin. The inferior 

 portion becomes suddenly narrow, and ends in a point. 

 It may be considered as consisting of a dermal plate, 

 concave on the central aspect, having its concavity fill- 

 ed up with layers which are convex on their central as- 

 pect. 



According to our observations the dermal plate ap- 

 pears to consist of three different lamina;, arranged pa- 

 rallel to one another. The external or dorsal layer is 

 rough on the surface, and marked by obscure concen- 

 tric arches towards the summit, formed by minute 

 knobs, which become larger towards the base, where 

 they appear in the form of interrupted transverse ridges. 

 It is uniform in its structure, and the tubercles possess a 

 polish and hardness equal to porcellaneous shells, al- 

 though they blacken speedily when put in the fire, and 

 contain a good deal of animal matter. On the central 

 side of this layer is one flexible and transparent, simi- 

 lar to horn, and smooth on the surface. The third lay- 

 er is destitute of lustre, and in hardness and structure 

 resembles mother of pearl shell*. 



The layers which fill the concavity of this deimal 

 plate arc slightly convex on the central aspect, and are 



