THE LIVING WORLD 



237 



FIG. 51. 



an arthropod. Instead of a succession of jointed limbs, 

 the animal moves upon a single, elongated, muscular 

 expansion, called the foot. Indeed, serial symmetry is 

 almost abolished, the viscera being arranged in a spirally 

 coiled projection upwards from the foot, and this projec- 

 tion is protected by a similarly coiled calcareous shell. 

 There is a well-developed head, 

 with two pairs of retractile pro- 

 cesses, the larger of which bear 

 eyes. On the right side of the 

 animal, close to the anterior 

 edge of the shell, is a large 

 aperture which is that of the 

 breathing sac. There are no 

 jaws, like those either of a ver- 

 tebrate or an arthropod, but 

 within the mouth there is a 

 crescent - shaped plate above, 

 and a cartilaginous cushion or 

 pad, bearing teeth, below. 



The nervous system is in the 

 form of a ring of nervous 

 tissue round the gullet with 

 ganglia, whence nerves proceed 

 to a third pair of ganglia. 



The cuttlefish is like a snail 

 devoid of an external shell and 

 with the body not spirally 

 coiled, while the margins of the foot are drawn out into 

 long sucker-bearing arms, the eyes being sessile. It 

 is furnished with a pair of gills formed somewhat like 

 those of the lobster, and the main ganglia of its nervous 

 system are protected by a sort of cartilaginous skull. 



The oyster, mussel, and their allies mainly resemble 



CUTTLEFISH (Sepia). 



