462 THE ANATOMY OF INYERTEBRATED 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 TricJiocephalus acetabularls bj Delle 



Fig. V^.—Arrjonaufa argo. 

 1. 2. 3, 4, the other arins 

 the htfctocotylus detached. 



P>. male, with thehectocotylized arm ii.'cloped in its sac; 

 f the ri<rht side : aud 1' , 2', 4', ihosc ut' the leJt side. A. 



Cliiaje, 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^^ one 

 or other arm is peculiarly modified, but does not become 

 detached or serve as a receptacle for the spermatophores. 



The Deccipoda 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 sppiostaire, a pliraymocone, or 

 a combination of the latter with a pen. 



1 Steenstrnp. " Die Ileotocotylenbilduiiir lici Argovaxda nnd Trcmodopvs 

 erkliirt durch Reobachtuntrcn iihnlielier Bildimgeu bei deu Cephalopoden." 

 (" Archiv fiir Naturgescliiclite," 1850.) 



