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. 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 

 ca'ses the arms are 

 united by a web-like 

 fold, the interbrachial 

 membrane (Fig. 700) 

 which may reach 

 nearly to their ex- 



FIG. 698. Argonauta argo, female, showing the relations tremities. 



of the animal to the shell in the living state, the arrow Jj} the T 6 t T a - 



showing the direction of movement. /. funnel ; m. mouth, , , . , 



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

 it; wa, webbed arm clasping the shell. (From Coolie, r r .ji^j 



alter Lacaze-Duthiers.) Ol groups Ot slender, 



FIG. 697. ZiOligo vulgar is. 



view ; B, horny internal 

 Keferstein.) 



A, entire animal, dorsal 

 shell or pen. (From 



