Archive image from page 534 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana01todd Year: 1836 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 cree
Archive image from page 534 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana01todd Year: 1836 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 o
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