THE CEPHALOPODA. 



155 



This leads to the consideration of the class characters by which such varied forms of shell- 

 bearino 1 and shell-less animals can be known and recognised when found. Unlike those of some 

 Mollusca, the organs of the Cephalopoda are symmetrically arranged, having their right and left side 

 equally developed. The shells, too, of those forms which possess such an external covering, also grow 

 symmetrically. But only two among all the existing representatives have any external shells, namely, 

 the Nautilus and the Argonaut, all the rest are termed "naked" Cephalopoda, because they have only 

 an internal shell entirely hidden within the soft parts of the animal's body. They have a distinct 

 head, upon which, and around the mouth, are placed the principal appendages of the body in the form 

 of a circle of muscular arms or tentacles. These members fulfil the office alike of seizing and 

 holding the prey, and also act as organs of locomotion ; hence the name " head-footed " given to the 

 class. The free-swimmers, such as the Squids, Calamaries, and Cuttle-fishes have fins, which aid them 

 in progression through the water, but all rapid movement is effected by the forcible expulsion of the 

 water through the funnel from the respiratory chamber. Their progress, indeed, is effected stern- 

 foremost, as in the case of a rocket, the backward discharge in both instances being the cause of their 

 onward progress. 



The typical forms of the Cuttle-fishes were known and described by Aristotle more than 300 

 years B.C., but it remained for Professor Owen to point out the existence of two distinct and separate 

 orders of Cephalopods, clearly characterised by their respiratory organs, and to demonstrate how 

 inseparably this organisation was connected with the condition of the two types, the free-swimming 

 Cuttle-fishes on the one hand, and the sluggishly-crawling Nautilus on the other. 



Among the great groups of animals already described, various leading modifications of structure 

 were specially noticed, by seizing on which naturalists have been enabled to classify them readily ; so 

 also in the Cephalopoda, one group representing the Squids, Cuttles, Calamaries, the Argonaut, and 

 the Octopus were found to lead very active lives, and to be excellent swimmers, and as they 

 had only two gills, Professor Owen called them " Dibranchiata." The other group limited 

 nowadays to the Pearly Nautilus, but formerly, as will be seen anon, quite a dominant class in 

 the seas and oceans of former ages Professor Owen found to possess four gills, and named them 

 "Tetrabranchiata." They were rather sluggish in their habits, as compared with their modern class- 

 mates, proving clearly that habit and function are directly co-ordinate with one another. 



All the Cephalopoda are marine and carnivorous, and possess considerable locomotive 

 power. At the bottom of the sea they can walk about head downwards, by means of the 

 tentacles which surround the mouth, and which are usually provided with numerous suckers or 

 " acetabula."* They are also able to swim, partly by the aid of lateral expansions of the integu- 

 ment or by fins, but chiefly, as has been already stated, by the forcible expulsion of water through 

 the tubular ex-current funnel from the respiratory chamber, in which the two or four plume-like 

 gills are placed. 



The mouth is armed with powerful jaws, resembling in form, texture, and position the man- 

 dibles of a bird, being especially like a parrot's beak in shape. The tongue is large and fleshy, and, in 

 part, seems to be endowed with the organs both of touch and taste, 

 and, in part, it is armed as in the Garden Snail, and the Rock 

 Limpet, and other Gasteropoda with recurved spines or teeth. 



But the eyes are perhaps the most striking organs in these 

 creatures, being both large and brilliant, and well express the keen 

 activity and alertness for which the majority of this wonderful group 

 are conspicuoiis. 



It has already been noticed that nearly all the existing forms of Cephalopoda belong to the naked- 

 bodied, or internal-shelled, section the two-gilled (or Dibranchiata). Members of this division cannot 

 rely upon the protective covering of their shells as the Garden Snail does, but like the Garden Slugs, 

 many of which, we shall presently see, have small rudimentary internal shells, they have to rely on 

 cunning, or greater activity, and the substitution of other means of escape and defence, than those which 

 a strong external shell would have afforded. They possess powerful tentacles, furnished with suckers. 



* Acetabulum (pi. acetalula), Lat., a cup, a calyx: a term applied to the suckers or "cups" on the arms of the 

 Cuttle-fish and other Dibranchiate Cephalowods, which have been hence termed Acetalulifera. 



TONGUE OF THE OCTOPUS. 



