156 



MANTTAL OF THE MOLLTJSCA . 



mandibles of birds. Tlie tongue is large and flesby ; part of its 

 surface is sentient, whilst the rest is armed witb recurved spines ; 

 tbeir eyes are large, and placed on tbe sides of tbe bead. In all 

 probability tbey possess tbe faculty botb of smelling and bearing. 

 All are carnivorous, and live in tbe sea. 



Tbe nervous system is more concentrated tban in tbe otbcr 

 moUnsca, and tbe brain is protected by a cartilage. Tbe respira- 

 toiy organs consist of ivro or four plume-like gills, placed 

 symmetrically on tbe sides of tbe body, in a large brancbial 

 cavity, opening forwards on tbe under* side of tbe bead : in tbe 

 middle of tbis opening is placed tbe siphon or funnel. Tbe sexes 

 are always distinct. Tbe cephalopoda are divided into two 

 orders, tbo names of wbicli are derived from tbe number of tbe 

 IrancMce. 



Order I. — Dibranchiata, Owen. 



Animal swimming; naked. Head distinct. Eyes sessile, 

 prominent. Mandibles borny (PI. I., fig. 2). Arms eight or 

 ten, provided with suckers. Body round or elongated, usually 

 with a pair of fins ; hranchice two, furnished with muscular 

 ventricles ; inh-gland always i3resent ; funnel a complete tube. 



Shell internal (except in argonauta), horny or shelly, with or 

 without air-chambers. The shell of the argonaut does not 

 correspond with the ordinary shell of mollusks. (See p. 39.) 



The tj^ical forms of the cuttle-fishes were well described by 

 Aristotle, and have been repeatedly examined by modern 

 naturalists ; yet, until Professor Owen demonstrated the exist- 

 ence of a second order of cephalopods, departing from all the 

 above-mentioned characters, it was not clearly understood how 

 inseparably the organisation of the cuttle-fishes was connected 

 with their condition as sivimming mollnsca, breathing hj tiuo 

 gills. There are two types of lung structure among the dibran- 

 chiates. Thus, in Octopus and Sepia the gills form a cylinder, 

 while in Loligo and other genera tbey form a haK cylinder. 



The characters which co-exist with the two gills, are tbe 

 internal rudimentary shell, and the substitution of other means 

 of escape and defence, than those which an external shell would 

 have afforded ; viz., jDowerful arms, furnished with suckers ; the 



* According to the established usage, we designate that the vnder or ventral side of 

 the body, on which the funnel is placed. But if the cuttle fishes are compared with 

 the nucleobranchs, or the nautilus with the holostomatous gasteropods, their external 

 analogies seem to favour an opposite conclusion. There are many teiTas in use which 

 are apt to mislead, such as flns, arms, &c. ; they have a definite meaning when applied 

 to the vertebrata, but not so when applied to the invertebrata. 



