328 MOLLUSCS. 



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

 smooth, if irritated, become suddenly covered with conical tubercles or more or less 

 elongate cirri. 



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

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

 upon it suddenly. Like every other group of animals, they have their enemies, 

 being 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 prob- 

 ably exceed 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 internal or external shells which have been preserved ; and 

 what proportion of all the forms that have existed in all times do the fossilised 

 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 representatives. The entire order of the 

 Ammonoidea, which contains the well-known discoid, convoluted, chambered 

 Ammonites, is entirely extinct, and it is a matter of uncertainty 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 fossilised remains of 

 octopods have been identified with certainty, and the Spirilla of to-day, which 

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



Two-Gilled Group, — Order Dibranchiata. 

 Octopus Tribe, Suborder Octopoda, — Family Octopodid.e. 

 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 octopods comprise several families distinguished by differences in the general 

 build, the presence or absence of lateral fins, the number of rows of suckers on 

 the arms, variations in the radula, etc. Since the establishment of public aquaria, 

 in comparatively recent years, most persons have had an opportunity of seeing 

 the unsightly octopus in its native element. An unpleasant, forbidding creature 

 it is, contracting and swelling, or looking like a shapeless but living mass. We 

 observe the eight tapering arms, with the two rows of suckers along the inner 

 side of each, numbering: about two thousand altogether in some individuals. We 

 note the two staring eyes which seem ever on the watch, the funnel often exposed 

 to view, and the mottled skin. About ninety species of octopus are known, which 



