CEPHALOPODA. 5 



which, in the tetrabranchiate Ccphalopod, is supplied by a multitude of tcntacula 

 grouped around the mouth. These arms or tcntacula are organs as well of locomotion 

 as of touch and prehension. In the dibranchiate Ccphalopod the arms are furnished 

 with suckers (act-tabula), and arc of two kinds, viz. : eight sessile arms encircling the 

 mouth, and connected at the bases by a muscular web more or less broad ; and two 

 tentacular arms placed one on each side, and capable of considerable extension. The 

 Octopods arc furnished with the sessile arms only ; the Decapods possess also the 

 tentacular arms. The development of the sessile arms appears to be in inverse ratio 

 with the retro-swimming power of the animal, and, consequently, as we have before 

 seen, with the size of the body. In the pelagic Decapods, which possess the highest 

 retro-swimming power, and whose body is comparatively large, the arms are short ; 

 while in the finless Octopods and the littoral Decapods, which have small bodies, and 

 are consequently bad swimmers, and whose habits require the means of creeping along 

 the ground, the arms are infinitely larger, and the connecting web is broader, so that 

 they serve also for reptation. 



The arms, to adapt them more perfectly for prehensile purposes, are provided with 

 suckers placed in serie, on the inner surface. These are sometimes simple, i. e. 

 unarmed ; but in some genera they are surrounded by a horny dentated hoop, and in 

 others are uncinated, or armed with sharp, horny hooks. When the prey is once 

 seized by this formidable apparatus, escape is hopeless. In the tetrabranchiate 

 Cephalopod, which is always attached to a dense calcareous shell, and whose principal 

 food appears to be the Crustacea or testacea living at the bottom of the sea, the 

 complicated mechanism of the arms entirely disappears, and the animal is provided 

 with numerous, small, retractile tentacles, by which the sense of touch, as necessary to 

 it as enlarged vision is to the dibranchiate Cephalopod, is largely developed. 



The presence of the sucker bearing arms, or of the tentacula, is an ordinal 

 distinction, and has been adopted by the French naturalists for the designation of the two 

 orders, corresponding with the dibranchiate and tetrabranchiate orders of Professor 

 Owen, into which they have divided the Cephalopods ; the armed and unarmed 

 conditions of the suckers are also used as subordinal and generic distinctions, and 

 characters of families and genera are founded upon the retractile power of the 

 tentacular arms. 



Exclusive of the impulsion derived from the funnel, and the capacity to rise and 

 float in the sea which the chambered and siphoniferous shell affords, the tetrabranchiate 

 Cephalopod can only creep, like the gasteropods, along the bottom of the sea by means 

 of the free and expanded margin of the anterior extremity of the body. 



The animal whose zoological peculiarities have been thus cursorily noticed, is 

 sometimes lodged in a symmetrical shell, unilocular ox camerated (multilocular), that is, 

 presenting a series of chambers divided from each other by thin partitions (septa), and 

 successively added by the animal to meet the exigencies of its increasing bulk, and in 



