Cephalopoda. 



47 



structures which render it a very complex and efficient organ 

 comparable to that of the Vertebrata. Some of the arms are 

 modified in the male for the purpose of pairing, and in some of 

 the Oeiopoda the moditied tentacle becomes detached. 



A striking peculiarity of Jbhe class is the elaborate development 

 of chromatophores in certain families. These are pigment cells 



Fig. 40. 



A, the upper, B, the lower beak of Architeuthis monachus ; 

 one-third natural size. 



Fig. 41. 



The Common Octopus (Polypus vulgaris), resting. 



of various colours found in the skin and furnished with muscle 

 fibres and a nerve supply. When stimulated by some shock or as 

 the result of direct control by the nervous system the chromato- 

 phores of the same colour may all be contracted or extended. 

 The colour of the animal is thus susceptible to rapid change and 

 may be adjusted for purposes of protection. 



All the Cephalopoda are marine, active, and carnivorous. They 

 are divided into two orders — Tetrabranchia and Dibranchia. 



