3 28 
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 Spirula 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 OCTOPOLIDyE . 
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 
