xn 



PHYLUM MOLLUSC A 



791 



gress 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 

 ninnel by the contraction of the muscular mantle (Fig. 687). 

 When fins are present they 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. 688) ; 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 



Flo. 687.— 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.) 



Argonauts (Fig. 689) 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 capable of being partly 

 or entirely retracted within certain sacs situated at their bases. 

 In nearly all one of the arms is specially modified (or heclocotylised) 

 to act as an intromittent organ. This modification is only very 

 slight in Sepia and confined to the base, and is most marked in 

 certain of the Octopoda (Fig. 690), including the Argonauts. In 

 the latter, before the breeding season, the third arm in the male 

 is found to be represented by a rounded sac, which subsequently 



