CEPHALOPODA. 



519 



two strong lateral muscles (d*\ which are in- 

 serted into the walls of the last chamber. 

 The numerous hollow arms (e, e) and retrac- 

 tile tentacles (J\J') y mentioned in the general 

 characters of the class are peculiar to this order, 

 and the head is further provided with a large 

 ligamento-muscular plate, or flattened disc, (g,) 

 which, besides acting as a defence to the open- 

 ing of the shell, serves also, in all probability, 

 as an organ for creeping along the ground, 

 like the foot in the Gasteropods. There are no 

 fins or analogous organs for swimming. 



The jaws of the Tetrabranchiata are strength- 

 ened by a dense, exterior, calcareous coating, 

 and have thick dentated margins. 



The eyes are pedunculated (h,fg. 205) and 

 of a simple structure. 



There is no organ of hearing. 



The gills are four in number, and without 

 branchial hearts. 



The circulating system is provided with but 

 one ventricle, which is systemic or propels 

 arterial blood. 



There is no ink-bag. 



The inferior parietes of the funnel (i, jig. 

 205) are divided longitudinally. 



Order II. DIBRANCHIATA. Syn. Cryp- 

 todibranehes, Blainv.; Acetabuliferct) D'Orb. ; 

 plus the SpirulidfE and Belemnitida. 



In the Dibrunchiute Cephalopods one genus 

 alone ( Argonauta, Jig. 206) has been hitherto 

 found in which the body is protected by an 



Tig. 206. 



The Paper Nautilus or Argonaut, 

 Argonauta Argo, Linn. 



external shell (a) ; but this, though symme- 

 trical, and convoluted on a vertical plane, 

 consists of one simple chamber, or is ' mono- 

 thalamous,' and does not adhere to the body 

 of its Cephalopodous occupant, either by a 

 hydraulic pipe or lateral muscles. All the 

 other genera of the Dibranchiate Order are 

 naked ; but they are provided either with an 

 internal siphoniferous polythalamous shell,f or 

 the remains of a shell are found in various stages 



* The letter is placed on the portion of 

 broken shell which still adhered to one of the 

 lateral muscles in the specimen taken by Mr. Geo. 

 Bennett, in the New Hebrides Islands. See PI. 1. 

 Memoir on the Pearly Nautilus, 4to. 1832. 



t This is the case in the Families Spirulidee and 

 Belemnitidee; the terms Polythalamacea or Sipho- 

 nifera, therefore, do not distinguish the preceding 



of degradation lodged in the substance of the 

 dorsal part of the mantle. 



The arms of the Dibranchiata are, properly 

 speaking, eight in number, (c, 1, 2, 3, 4, 

 Jig. 206,) to which, in many genera, two longer 

 tentacles (d, d,jigs. 207, 208) are superadded. 

 Both kinds of prehensile organs are provided 

 with acetabula, or suctorious discs for adhesion ; 

 and hence the order has been termed Acctabu- 

 lij'era. 



The jaws are horny, and their margins tren- 

 chant. 



The eyes are sessile, (e, e,fg. 207,) and of a 

 more perfect structure. 



The organ of hearing is distinctly developed. 

 The gills never exceed two in number ; but 

 the branchial circulation is aided by two mus- 

 cular ventricles, situated one at the base of each 

 gill ; hence there are three distinct hearts in 

 this order. 



There is an organ for secreting and expelling 

 an inky fluid, used as a means of concealment. 

 The parietes of the funnel are entire, (i, 

 Jigs. 206, 208.) 



Subdivision of the Orders. In the ancient 

 periods of the globe the Tetrabranchiate Cepha- 

 lopods appear to have abounded in every sea; one 

 genus only, however, viz. the Nautilus, appears 

 to have escaped the influences which have ren- 

 dered extinct the rest of this once extensive order. 

 Their chambered shells are found, generally in 

 a fossil state, in all the regions of the globe, 

 and at every elevation, charac- 

 terizing the strata of the se- 

 condary formation. In some 

 places they occur in such pro- 

 digious numbers that the rocks 

 appear to be composed almost 

 exclusively of their remains. 



Some of these fossil shells 

 testify the immense size to 

 which their animal construc- 

 tors must have attained : the 

 shells called ' Cornua Am- 

 monis,' which were formed by 

 Cephalopods resembling the 

 Nautilus, have been found 

 measuring four or five feet in 

 diameter; some of the straight 

 chambered shells, called ' Or- 

 thoceratites,' exceed four feet 

 in length; other species again appear not to 

 have surpassed the size of a grain of rice. 



As the consideration of these remains, of 

 which the Tetrabranchiate division of Cepha- 

 lopods is almost exclusively composed, would 

 necessarily oblige us to exceed the limits allot- 

 ted to this article, we shall here subjoin merely 

 the characters of the two families into which 

 they naturally resolve themselves, and to which 

 their distribution appears to be limited. 



or Tetrabranchiate Order, nor indicate any cha- 

 racters peculiar to that group, or which are of ordi- 

 nal importance : and in other Molluscous classes 

 it may be observed that modifications of the shell 

 fail to afford indications of the primary divisions, 

 which are uniformly based, as in the present arrange- 

 ment of Cephalopods, on the modifications of the 

 respiratory system. 



