170 ZOOLOGY. 



poulpe (octopus) there are eight of these appendages, and in 

 the sepia ten ; sometimes two of these enlarge like oars or 

 membranous veils, as in the argonaut (Fig. 449), or elon- 

 gate so as to become filiform, as in the calmar (Fig. 446), 

 and especially in the calmaret (Fig. 8). In the tetrabran- 

 chiate cephalopodes these appendages are all slender, without 

 suckers, and extremely numerous (Fig. 450). 



604. Most of the mollusca of this class are remarkable 

 for the development and perfection of their eyes, which 

 greatly resemble those of vertebrata. Several have also an 

 auditory apparatus, but this organ is reduced to a small 

 membranous sac, receiving a nerve. 



Finally, the nervous system of these animals is more com- 

 plex than in the other mollusca, and the various ganglions 

 grouped around the gullet have a tendency to coalesce into a 

 single mass. The medullary collar so formed is composed of 

 a pair of cephalic ganglions, whence arise the optic nerves, 

 &c. ; of a pair of ganglions, situated more forward, but under 

 the gullet, and furnishing the nerves of the tentacula (Fig. 

 451) ; finally, of a pair of thoracic ganglions, giving origin 

 to the nerves of the mantle, and to two cords which, proceed- 

 ing backwards, form on each side of the abdomen, a ganglion, 

 from whence arise the branches destined for the heart, gills, 

 &c. 



605. All the cephalopodes are marine; they are very 

 voracious, and live chiefly on Crustacea and fishes, which 

 they seize with their supple and vigorous arms, and whose 

 flesh they easily devour by means of their sharp mandibles. 

 Some of these animals are lodged in shells turned on them- 

 selves : such are the argonaut and the nautilus ; but some 

 naturalists think that the first does not itself form the cal- 

 careous covering, but lives as a parasite in the shell of another 

 mollusc. 



This class comprises the octopus (Fig. 162), the argonaut 

 (Fig. 449), the sepia, the calmar (Fig. 446), the calmaret 



the gullet anew, and to form there a small anterior ganglion, whence arise 

 the labial nerves ; b, tentacular ganglions, whence arise the nerves of the 

 arms ; o, optic nerves, arising from the lateral parts of the brain, and soon 

 swelling into a large ganglion; t, small venous tubercles, situated on the 

 origin of the optic nerves ; g, cesophageal or ventral ganglion ; r, great 

 visceral nerve, one of the branches of which has on it an elongated ganglion 

 (r), and penetrates into the gills ; m, nerves which arise also from the post- 

 (Bsophageal ganglions, and which have on their passage a large star-shaped 

 ganglion (e), whose branches proceed to the mantle. 



