462 THE AXATOMY OF INVERTEBRATED ANIMALS. 



semen is detached and left in the mantle cavity of the female 

 (Fig. 133, A). When first discovered it was regarded as a 

 parasite, and termed Trichocephalus acetabular is by Delle 



Fig. \Z^.— Argonaut a argo.—B, male, with thehectocotylized arm inclosed in its sac; 

 1. 2. 3, 4, the other arms of the right side ; and 1', 2', 4', those of the left side. A, 

 the hectocotylus detached. 



Chiaje, while the corresponding body found in an Octopus 

 was called Hectocotylus octopoclis by Cuvier. 



In Tretnoctopus, it is the third arm on the right side 

 which becomes the Hectocotylus. In other Octopocls,^ one 

 or other arm is peculiarly modified, but does not become 

 detached or serve as a receptacle for the spermatophores. 



The Decapocla have ten arms, two of which are usually 

 much longer than the rest, and can be protruded from, or re- 

 tracted into, sockets. The acetabula have horny rims, which 

 may take on the form of hooks. 



Hectocotylization does not go further than a modification 

 of the form of one of the arms. There is always an internal 

 shell, which is either a pen, a sepiosfaire, a pliragmocone, or 

 a combination of the latter with a pen. 



1 Steenstrup, " Die Hectocotylenbilduna: bei Argonavta utid Tremoctopus 

 erkliirt durch Beobachtunsren aiinlicher Bildungen bei den Cephalopoden." 

 (" Archiv fur Naturgeschichte," 1856.) 



