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 acetabularls bj Delle 



Fig. ISS.—Arf/onaufa argo.—^, 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 kit side. A, 

 the hectocotylus detached. 



Chiaje, while the corresponding body found in an Octopus 

 was called Hectocotylus octopodis by Cuvier. 



In TremoctopKS^ it is the third arm on the right side 

 which becomes the Hectocotylus. In other Octopods^ 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 sep)iostaire, a phragmocone, or 

 a combination of the latter with a pen. 



1 Steenstrup, " Die Hectocotylenbildunor bei Argovuuta und Tremodopvs 

 erl^lart durch Beobachtuncren ahnlicher Bildungen bei den Cephalopoden." 

 (" Archiv fiir Naturgeschichte," 1856.) 



