THE DIBRANCHIATA. 



461 



terior pair of arms are greatly expanded, and, being turned 

 back over the mantle, secrete an elegant &hclly structure 

 which covers the bodv, and serves for the attachment of the 



Fig. IZl.—Argonavfa a7'go.—A, female with the expanded anns in their natural 

 positiou, cuibracicg the shells,* d, the other six arms; a, the luunel. i^, ace- 

 tabula. 



Fig. lS2.—A7'gonauta argo, male, with the Hedocoti/lus-arm attached. 



eggs. In this genus, and in some other Octopods ( Octopus 

 carina, Tremoctopus violace^ff^^ nnd T. Quoyamis),\kiQV[\^Q 

 is very much smaller than the female, and gives rise to a 

 Ilectocotylus. 



In Argonauta argo (Figs. 132, 133), it is the third arm on 

 the left side which becomes thus modified. At first it has 

 the form of a sac, within which the slender terminal part of 

 the arm is coiled up (Fig-. 133, B). The sac splits to give 

 exit to the latter (Fig. 132), and its two halves reunite on the 

 outer face of the base of the arm to form a chamber, which 

 becomes filled with spermatophores in a manner not yet un- 

 derstood. During sexual union the arm thus charged with 



