246 NAKED CEPHALOPODA PABT m. 



below the tentacles. All the naked Cephalopods propel 

 themselves back foremost in the sea, by the forcible 

 expulsion of water from the gill chamber through this 

 siphon. 



The head protrudes from the top of the mantle ; it 

 has a pair of large eyes on sockets, and some species of 

 these animals have eyelids. The ears are cavities under 

 the cartilage of the skull, containing a small sac and an 

 otolite. The mouth, which is terminal, and surrounded 

 by the tentacles, has powerful jaws like a parrot's beak 

 reversed, acting vertically. The tongue is large, the 

 posterior part is covered with recurved spines, and the 

 organ of smell is a cavity near the eyes. The Naked 

 Cephalopods are remarkable for having three hearts, or 

 propelling vessels one for the circulation of arterial 

 blood through the body, the others for projecting venous 

 blood through the gills, at whose base they are situated. 

 The arms or tentacles of all the Naked Cephalopoda 

 are formidable organs of defence and prehension, but 

 are most powerful in the Loligo vulgaris, the Poulpe, 

 and the Cuttle, on account of one pair of the tentacles 

 being long slender arms, dilated at their extremities 

 into flat clubs. On the inner surface of each of the 

 tentacles, and upon the lower surface of the dilated ex- 

 tremities of the long ones, there are multitudes of suck- 

 ing disks, which, once fixed to an object, adhere so 

 firmly that it is easier to tear off a portion of the ani- 

 mal's tentacle than to make it release its hold. These 

 sucking disks, which are placed in parallel rows, are 

 represented magnified in fig. 180. Each sucker consists 

 of a firm cartilaginous or fleshy ring (e), across which a 

 disk of muscular membrane (/) is stretched, having a 

 circular opening (g) in its centre. A cone-shaped mass of 

 flesh fills the opening like a piston, capable of being 

 drawn backwards ; the membranous disk can also be 

 drawn in. When one of these sucking-disks touches 



