104 A NATURAL HISTORY OF THE SEAS 



abound in all seas, and, whilst some ate valued for food, 

 the group generally is regarded with repugnance. These 

 molluscs date back to the remote geologic past when the 

 sea swarmed with many forms long since extinct. 



In general structure a Cephalopod mollusc is much more 

 highly organized than any of the preceding groups. The 

 body is baglike and may be reinforced by an exterior or 

 internal shell. The head is usually marked off by a 

 distinct neck and bears a pair of large well-formed eyes 

 and a brain enclosed in a cartilaginous case. A horny 

 beak made in two pieces is present and is superficially 

 very like a parrot's. These animals' chief claim to dis- 

 tinction, however, is the foot, which is split into eight 

 or ten arms studded with one or two rows of circular 

 suckers. The arms encircle the mouth and when ten 

 arms are present the two longest are coiled away into pouches 

 one on either side of the head, ready to be shot forward 

 for seizing prey as required. 



All Cephalopods are active and carnivorous, and though 

 often aggressive, their soft bodies render them very 

 vulnerable and they must resort largely to concealment or 

 swift retreat for safety. In all species the skin is lavishly 

 endowed with pigment cells of various tints and these 

 being contracted or dilated at will, provide a constant 

 succession of colours, which often exactly harmonise with 

 the animal's surroundings. In addition, all can secrete 

 large quantities of a dark inky fluid or sepia, which is 

 stored in a special sac as required, and shot out through 

 the syphon when occasion rises, obscuring the water 

 for some distance round and effectually covering the 

 creature's retreat. 



