XII 



PHYLUM MOLLUSCA 



769 



of being partly or entirely 

 retracted within certain 

 sacs situated at their 

 bases. In nearly all one 

 of the arms in the male 

 is specially modified (or 

 hectocotylised) 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. 

 699), including the Argo- 

 nauts. In the latter, 

 before the breeding 

 season, the third arm in 

 the male is found to be 

 represented by a rounded 

 sac, which subsequently 

 bursts and sets free the 

 elongated hectocotylised 

 arm. Spermatophores 

 are taken by the arm 

 from the genital opening, 

 and in the act of copula- 

 tion the entire arm is 

 detached and left in the mantle-cavity of the female. 





FIG. 697. Loligo vulgaris. 



view ; B, horny internal 

 Keferstein.) 



A, entire animal, dorsal 

 shell or pen. (From 



"CSfip 



FIG 698. Argonauta argo, female, showing the relation- 

 of the animal to the shell in the living state, the arrow 

 showing the direction of movement. /. Funnel : m. month, 

 with jaws projecting; sh. shell, with arms as seen through 

 ' it ; wa, webbed arm clasping the shell. (From Cooke, 

 after Lacaze-Duthiers.) 



In other 



cases the arm is not 

 detached. The 

 suckers are some- 

 times stalked, 

 sometimes sessile, 

 sometimes armed 

 with hooks, some- 

 times replaced by 

 hooks. In many 

 cases the arms are 

 united by a web-like 

 fold, the interbrachial 

 membrane (Fig. 700) 

 which may reach 

 nearly to their ex- 

 tremities. 



In the Tetra- 

 branchiata the series 

 of groups of slender, 



