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. 131. — Argonauta argo.—A, female with the expanded arms in their natural 

 position, embracing the shell b; d, the other six arms; a, the funnel. B, ace- 

 tabula. 



Fig. 132.— Argonauta argo, male, with the Hectocotylus-&vm attached. 



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

 carina, Tremoctopns v iolaceu :s, and T. Quoyanus), the male 

 is verv much smaller than the female, and p-ives rise to a 

 Hectocotylus. 



Tn 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 (Fio;. 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 



