CEPHALOPODA 515 



the sides. Round the mouth are the large and mobile tentacles (arms) 

 for seizing prey which are often considered to be part of the foot. At 

 one side, generally called posterior, is the mantle cavity, and protruding 

 from its opening is the funnel, which is the remaining part of the foot. 

 The visceral hump is the conical apex of the animal. Instead then, of 

 being protrusible like that of a lamellibranch or used for gliding like 

 that of a gasteropod, the main part of the cephalopod foot is greatly 

 modified for respiratory purposes. In view of the fact that there is 

 no boundary between the head and the foot in molluscs, discussion 

 as to whether the tentacles are part of the head or the foot is 

 difficult and unimportant. 



The shell has become internal and is a rather substantial plate which 

 acts as an endoskeleton. The absence of a rigid envelope has made it 

 possible for the mantle to become very mobile and to develop thick 

 muscular layers, circular muscles running round the mantle cavity 

 and longitudinal running towards the apex of the hump. When the 

 latter contract and the former relax the mantle cavity enlarges and 

 draws in water which circulates round the ctenidia ; when the reverse 

 action takes place the first effect of the contraction of the circular 

 muscles is to draw the mantle lobe tight round the neck and then, 

 when the contraction reaches its height, the water is expelled through 

 the funnel. In rest these movements are gentle and rhythmic and 

 only effect the change of water necessary for respiration. At the same 

 time the animal is usually swimming slowly forward by the undula- 

 tory movement of the lateral fins. But if Sepia is alarmed or excited 

 the muscles contract violently and the spasmodic ejection of water 

 through the funnel causes the animal to dart quickly backwards. Not 

 only is the mantle highly muscular but the dermis contains large 

 cells filled with pigment, the chromatophores , which can be dilated 

 by the contraction of radiating muscle fibres attached to the cell wall. 

 By alternate contraction and expansion of the chromatophores, waves 

 of colour are made to pass rapidly over the surface of the animal. The 

 colour change which is brought about in this way is to a certain extent 

 a response to the character of the background. 



Sepia swims with the longest axis horizontal, the upper flattened 

 surface is that under which the shell lies and the lower the mantle- 

 cavity surface. It is proposed to call these surfaces dorsal and ventral 

 respectively. All round the mantle in the horizontal plane rises a 

 horizontal fin by which the gentler swimming movements are effected. 



When the mantle cavity is opened as shown in Fig. 378, the 

 funnel is seen with its narrow external and wide internal openings, and 

 at the base of it two sockets which fit corresponding knobs on the 

 mantle. This locking arrangement ensures that the mantle fits tightly 

 on the neck and so that all water is expelled by the funnel. At the 



33-2 



