.3296 THE MOLLUSKS OR SHELLFISH 



distance up the arms, and forming a sort of umbrella, which is doubtless of use in 

 the capture of their prey. In the nautilus the arms are different, being short, 

 pointed tentacles, unprovided with suckers. 



The nervous system is more developed than that of other mollusks; it is con- 

 centrated around the gullet, and protected by a cartilaginous plate, a sort of rudi- 

 mentary skull. The skin of the naked cephalopods is more or less thickly studded 

 with points or dots of various colors. These pigment cells are subject to alter their 

 tint at the will of the animal, which, chameleon-like, assumes very different aspects. 

 Some species also, which, when in repose or undisturbed, have the outer skin 

 smooth, if irritated, become suddenly covered with conical tubercules or more or 

 less elongate cirri. 



Cephalopods are very voracious, feeding on fishes, mollusks, and crustaceans. 

 Some species pursue and capture their prey, while others lie in wait and pounce 

 upon it suddenly. L,ike every other group of animals, they have their enemies, be- 

 ing devoured in enormous quantities by cetaceans, fishes, and sea birds. In some 

 countries various species are esteemed as an article of food. Although about four 

 hundred and thirty species of living cephalopods have been described, some of these 

 are so inadequately defined, that the total, in round numbers, does not probably ex- 

 ceed about three hundred and eighty. These have been arranged in some seventy- 

 five genera and fourteen families. About half the genera contain but a single species 

 each, while nearly half the known forms belong to the three genera Octopus, Sepia, 

 and Loligo. The cephalopods of bygone ages far surpass in number those which 

 survive, and it is probable that we only know but a moderate proportion of the 

 forms that have passed away in the various geological epochs; for what idea have we 

 of the shell-less tribes which may have inhabited ancient seas, whose soft bodies 

 have decomposed at death, leaving not a vestige behind? It is only those with in- 

 ternal or external shells which have been preserved; and what proportion of all the 

 forms that have existed in all times do the fossilized remains known to us represent ? 

 The seas of our own times contain a large number of cephalopods, the existence of 

 which in past geological ages cannot be proved; but, on the other hand, we know of 

 great numbers of fossil genera and species of which there are no living representa- 

 tives. The entire order of the Ammonoidea, which contains the well-known discoid, 

 convoluted, chambered Ammonites, is entirely extinct, and it is a matter of uncer- 

 tainty whether they should be classed with the dibranchiate or tetrabranchiate 

 group, or be regarded as a distinct order by themselves. On the contrary, 

 although the probability is that many existed in bygone ages, only a few fossilized 

 remains of octopods have been identified with certainty, and the Spirula of to- 

 day, which occurs in countless thousands, also appears to be unknown in the past. 



GROUP Order DIBRANCHIATA 

 OCTOPUS TRIBE, Suborder Octopoda Family 



Commencing with the order Dibranchiata, we find this divided into the two 

 suborders, Octopoda and Decapoda, according to the number of arms. Of these the 



