462 THE ANATOMY 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. 133. — Arcjonauta 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 octopodis by Cuvier. 



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

 which becomes the Hectocotylus. In other Octopods, x one 

 or other arm is peculiarly modified, but does not become 

 detached or serve as a receptacle for the spermatophores. 



The Decapoda 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 hornv rims, which 

 may take on the form of hooks. 



Hectocotvlization 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 sepiostaire, a phragmocone, or 

 a combination of the latter with a pen. 



1 Steenstrup, " Die Heetocotylenbildunsr bei Argonauta und Trcmoctopvs 

 erklart durch Beobachtuntren ahnlicher Bildiingen bei den Cephalopoden." 

 (" Archiv fur Naturgeschielite," 1856.) 



