250 THE SEA SHORE 



are carnivorous species, feeding on other molluscs ; and the former 

 is said to bore through the shells of its prey with the prominent 

 beak of its shell. 



Murex may be readily distinguished by the prominent longi- 

 tudinal ridges of the thick shell, its rounded aperture, and by the 

 partly closed canal running through the beak. It is known to 

 fishermen as the Sting Winkle ; the other species is called the Bed 

 Whelk in some parts, and in Scotland is known as the Buckle. 

 Like the common whelk, it is dredged largely for the market, and 

 is said to be far more esteemed than the former, from which it may 

 be distinguished by the fusiform shape of the shell and the long 

 straight canal. 



We now pass to the last and highest class of the mollusca, 

 called the Cephalopoda because they have a number of arms 

 attached to the head, round the mouth. Unlike the majority of 

 molluscs they are bilaterally symmetrical : and are much more 

 highly organised, in some respects even making an approach to 

 the vertebrates. Thus they generally have an internal hard 

 structure, either horny or calcareous in structure, representing the 

 vertebral column, and the circulatory system consists of arteries 

 and veins, connected by minute capillaries. The corpuscles of the 

 blood are also similar in form to those of the vertebrates. Exter- 

 nally they are all naked, with the exception of the nautilus and 

 argonaut of the warmer seas. 



The arms, so characteristic of the class, are eight or ten in number, 

 long and muscular, and provided with numerous suckers by which 

 the animal can cling with remarkable tenacity. These suckers are 

 situated on the inner surface of the arms, and the disc of each one 

 displays a series of muscular fibres, all converging from the 

 circumference towards the centre, which is occupied by a softer 

 structure that works inwards and outwards like the piston of a 

 pump. Thus the suckers form a system of exhausting air-pumps 

 by which a vacuum can be produced, and the tenacity of the grip, 

 maintained by atmospheric pressure, is so great that the arms, 

 strong as they are, may be torn asunder by attempting to pull 

 them from their hold ; and yet the animal can release its grip with 

 the greatest of ease by simply releasing the pistons of its pumps. 



The cephalopods are further distinguished by their very large, 

 glaring eyes, situated on the sides of the well-formed head, and by 

 powerful jaws that work in a vertical plane, like those of the verte- 

 brates, but somewhat resembling the beaks of certain birds. The 



