768 



ZOOLOGY 



3. GENERAL ORGANISATION. 



SECT. 



The uniformity of structure among the Dibranchiate Cephalopoda 

 is very great, and, as already stated, Nautilus is the only living 

 member of the Tetrabranchiata, so that comparatively little has 

 to be said to supplement the descriptions of the two examples. 



External Features. The general external shape differs very 

 little in the different members of the Dibranchiata : the body in 

 some is more elongated than in others ; the degree of compression 

 likewise varies. Fins may be absent, and the animal may progress 

 entirely by creeping with the aid of the long arms, or by swimming 

 by the movements of the arms, or under the propulsion of a current 

 of water forcibly ejected through, the funnel by the contraction 

 of the muscular mantle (Fig. 696). When fins are present they 



FIG. 696. Octopus vulgaris. A, at rest ; B, in motion. /. funnel, the arrow showing 

 the direction of the propelling current through the water. (From Cooke, after Merculiano.) 



may take the form of a continuous lateral flap as in Sepia, but, more 

 usually, are of the nature of flattened lobes situated towards the 

 aboral extremity of the body (Fig. 697) ; in Ctenopteryx they have 

 the character of fringes of filaments. The arms vary in length 

 and proportions and in the form and arrangement of the suckers. 

 Eight arms are present in the Octopoda and ten in the Decapoda. 

 In the former group the Argonauts (Fig. 698) have, in the female, 

 one pair of arms (wa.) flattened and expanded at the extremities 

 for the secretion and support of the shell (sh.). In the Decapoda 

 one pair of arms, the fourth, is always specially modified, as in 

 Sepia, to act as prehensile appendages or tentacles usually capable 



