CLASSIFICATION 285 



CLASS V— CEPHALOPODA 



The cuttlefish are pecuHar and specialised molluscs of great 

 muscular development and activity, not easily related to the 

 other groups. The foot is modified as a funnel by and through 

 which water is expelled to drive the animal vigorously backwards. 

 The pelagic forms have horizontal lateral fins by which they 

 swim forwards. Alone among animals, except perhaps for man 

 and the great apes, the cephalopods have a head which is not the 

 first part of the body to meet the external world. It has a high 

 concentration of nervous tissue, and large and well-developed eyes. 

 Surrounding the brain there is a skull of a material resembling 

 cartilage and rich in collagen, which is also found stiffening the 

 fins and tentacles. The mouth has a radula and is surrounded by 

 eight or ten long muscular tentacles, possibly part of the foot. 

 The coelom is often well developed and the development is direct. 



Besides the cuttlefish, which have either an internal shell 

 {Sepia) or none {Octopus) the class includes Nautilus, with a 

 spiral-chambered shell, and many extinct forms (including the 

 ammonites) also with shells. 



