38o CEPHALOPODA DIBRANCIIIATA chap. 



The nerve-centres which control the chromatophores or pigment 

 cells, causing them to expand or contract, are found to connect 

 with the optic ganglia ; hence the changes of colour may be 

 regarded as a reflex result of the creature's visual perception of 

 its surroundings. 



Order Dibranchiata 



Cephalopoda with two symmetrical branchiae, funnel com- 

 pletely tubular, mouth surrounded by 8 or 10 arms furnished 

 with suckers or hooks, ink-sac and fins usually present, eyes with 

 a lens ; shell internal or absent. 



The Dibranchiata are not known from Palaeozoic strata, and 

 first appear {Bclemnites, Belemnoteuthis) in the Trias. Whether 

 they are to be regarded as derived from some form of Tetra- 

 branchiata, e.g. Orthoceras, or as possessing an independent origin 

 fiom some common stock, cannot at present be decided. They 

 attain their highest development at the present time. The 

 earliest representatives of the Order (the Pkragmopliora) possessed 

 a shell chambered like that of the Tetrabranchiata. These 

 chambered Dibranchiates rapidly reached their maximum in the 

 upper Lias and as rapidly declined, until at the close of the 

 Cretaceous epoch they were comparatively scarce, only a few genera 

 {Beloptera, Spirulirostra) surviving into Tertiary times. 



The ordinary Dibranchiate Cephalopod may be regarded as 

 consisting of two parts — (a) the head, in which are situated the 

 organs of sense, and to which are appended the prehensile organs 

 and the principal organs of locomotion ; (h) a trunk or visceral 

 sac, enclosed in a muscular mantle and containing the respir- 

 atory, generative, and digestive organs. The visceral sac is often 

 strengthened, and the viscera protected, by an internal non-spiral 

 shell. The ' arms ' which surround the mouth are modifications 

 of the molluscan foot (p. 200), and are either eight or ten in 

 number. In the former case (Octopoda) the arms, which are 

 termed ' sessile,' are all of similar formation, in the latter 

 (Decapoda), besides the eight sessile arms there are two much 

 longer ' tentacular ' arms, which widen at their tips into 'clubs' 

 covered with suckers. 



Eemarks have already been made on the generative organs 

 of Cephalopoda (p. 136 f.), the branchiae fp. 170), the nervous 



