CEPHALOPODA. 5 
which, in the tetrabranchiate Cephalopod, is supplied by a multitude of tentacula 
grouped around the mouth. These arms or tentacula are organs as well of locomotion 
as of touch and prehension. In the dibranchiate Cephalopod the arms are furnished 
with suckers (acetabula), and are of two kinds, viz.: eight sessi/e 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 are 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 zz 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 wncinated, 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 principa! 
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, wxi/ocular or 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 
