738 



ZOOLOGY 



SECT- 



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. 646.) 

 When fins are present they may not 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. 647); in Cten- 

 jf t opteryx 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. 648) 

 have, in the female,, 

 one of the pairs of 

 arms (wa.), flattened 

 and expanded at the 

 extremities for the se- 

 cretion and support of 

 the shell (sh.). In the 

 Decapoda one of the 

 pairs of arms, the 

 fourth, is always speci- 

 ally modified, as in 

 Sepia, to act as pre- 

 hensile appendages or 

 tentacles capable of 

 being partly or entirely 

 retracted within cer- 

 tain sacs situated at 



their bases. In nearly all one of the arms is specially modified 

 (or Jiectocotylised) to act as an intromittent organ. This modifi- 

 cation is only very slight in Sepia and confined to the base. 

 It is most marked in certain of the Octopoda (Fig. 649), in- 

 cluding the Argonauts. In the latter, before the breeding 

 season, the third arm in the male is found to be represented 

 by a rounded sac. This subsequently bursts and sets free the 

 elongated hectocotylised arm. Spermatophores are taken by 

 the arm from the genital opening, and in the act of copulation. 



FIG. 647. Loligo vulgaris. 



horny internal 



view ; B, 



Kefersteiii.) 



A, entire animal, dorsal 

 shell or pen. (From 



i 



