DIBRANCHIATA 445 



Other Cephalopoda. 



All the cephalopods are predatory animals and inhabit the sea. In 

 addition to the ten-armed species (Sub-Order Decapoda), there is 

 also a large number in which the two long tentacles are absent 

 (Sub-Order Octopoda). Among the latter, the Common Octopus, 

 (Octopus vidgaris), which is found on the coasts of Southern and Western 

 Europe, is not only the commonest, but also the largest and strongest, 

 species. This animal between the tips of its extended arms, which far 

 exceed in length the saccular body, sometimes measures more than 

 10 feet. It lurks in wait for prey in rock caverns or among stones, 

 surrounded by the shells of the molluscs and crustaceans which have 

 formed its meal. In absence of a suitable hiding-place, the creature 

 constructs one for itself by collecting stones with its arms. Whilst on 

 the watch for prey, the skin, which is at other times smooth, appears 

 wrinkled, and the colour exactly resembles the rocky nest in which the 

 creature is hiding. The arms embrace the unsuspecting victim like so 

 many snakes. At their base they are united by webbed membranes, so 

 that their basal portion resembles a sack, in which the prey is firmly 

 and securely shut in. Eemains of the internal shell of a family of 

 decapods long since extinct, the Belemnites, have been preserved in a 

 fossilized condition, being popularly known as " thunderbolts." 



Opposed to the above-named group of cephalopods, all of which are 

 provided with two branchiae, we have another much smaller division, in 

 which four branchiae are developed (Order II.: Tetrabranchiata). 



The only living representative of this order is the Pearly Nautilus 

 (Nautilus}. The animal is an inhabitant of the Pacific and Indian 

 Oceans, and constructs a shell coiled into a spiral like that of a snail. 

 This shell, however, is divided into a number of chambers by transverse 

 partitions, of which fresh ones are constantly formed as the growth of 

 the animal proceeds. The foremost of these chambers is occupied by the 

 animal ; the rest are filled with air, and traversed by a cord-like mem- 

 branous process of the body (the siphuncle). The weight of the shell is 

 much diminished by the air in the chambers. The Nautilus lives near 

 the bottom, and has only occasionally been taken at the surface. 



An extinct order of cephalopods, the Ammonites, the fossilized 

 shells of which have come down to us, were animals similar to the 

 nautilus in all essentials of form and structure. 



