THE DIBRANCHIATA. 



461 



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

 back over the mantle, secrete an elegant shelly structure 

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



Fig. 'ISl.—Argonaufa argo.—A, female with the expanded anns in their natural 

 position, embracing the shell 6; c?, the other six arms; a, the luunel. i^, ace- 

 tabula. 



FiQ. lZ2.—Argonauta argo, male, with the Hectocotylus-tivm attached. 



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

 carina^ Tremoctojnis violaceits^ and T. §?«oyanw5), the male 

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

 Hectocotylus. 



In Argonauta arr/o (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 (Fiff. 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 



