CEPHALOPODA 589 



(4) Tribe Myopsida. Decapoda with a cornea in the eye, a 

 simple gladius, specially elongated 4th pair of tentacles, not 

 retractile into pits; fins united posteriorly; shore forms. 

 Loligo (Fig. 411 D). 



Suborder Octopoda. Dibranchs with eight tentacles and a reduced 

 coelom. Octopus, Argonauta, Opisthoteuthis . 



Sepia officinalis^ is a shallow-water form, in which the shell has 

 become internal. The general disposition of the organs remains much 

 as it would be if the animal inhabited the last chamber of a shell like 

 that of Nautilus (cf. Fig. 402 A and B). The whole body is cylindrical. 

 At one end, which would have projected from the shell, is the head 

 with the mouth in the centre and the two relatively enormous eyes at 

 the sides. Round the mouth are the tentacles (arms) for seizing prey 

 which are often considered to be part of the foot. Four pairs of these 

 are short and stout and covered with suckers on their inner surface. 

 The fourth pair (counting from the dorsal surface) are long and can 

 be retracted into large pits at their base ; there are suckers only at their 

 free end. The left hand member of the fifth pair in the male is slightly 

 modified by suppression of the suckers. At one side, called pos- 

 terior, 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, dis- 

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

 difiicult and unimportant. 



The shell has become internal and is a rather substantial plate 

 which acts as an endoskeleton. The absence of a figid 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- 



^ This description of the structure and habits of Sepia applies generally to 

 all the well-known Decapoda. 



