ARGONAUTA ARGO AND THE II ECTOCOTYLT. 55 



I may here observe, that in one case the sac burst before my 

 eyes, along the inner, oral side, in consequence of the violent 

 movements of the Hectocotylus, when the process about to be 

 described took place in exactly the same manner as in other spe- 

 cimens which were artificially opened. 



It is observed, in the first place, that the thick end of the Hec- 

 tocotylus is fixed to the pedicle of the sac or forms it ; next, that 

 the membrane of the sac is perfectly distinct from the filiform 

 appendage and from the neighbouring parts of the sucker-bear- 

 ing body ; but that, upon the thick portion of the body, while 

 it leaves the sucker- side free, it is attached along the back behind 

 the suckers, and forms the covering of the silvery sac above men- 

 tioned. 



The so-called pigmented testis capsule of Kolliker, however 

 (as it is observed in the Hectocotyli which are found free upon 

 female Argonauts), does not yet exist, and is subsequently formed 

 from the membrane of the sac. 



As soon indeed as the appendage and the thinner part of the 

 body, which usually become twisted upon their axis at the same 

 time, are evolved, the thick part bends forcibly back in the op- 

 posite direction to the previous curvature, that is towards the 

 back. By this means the longitudinally cleft membrane of the 

 sac is inverted, so that its inner surface comes to be exterior, and 

 the edges of the torn part are turned back towards the back of 

 the Hectocotylus, which is now concave. The previously external 

 pigmented layer of the sac now lies in the pit between these 

 edges, and when the latter have united, there is left only a 

 small cleft, a process which can naturally not be directly traced. 

 We have just such a pigmented capsule formed as has been already 

 found in the dorsal crest of Hectocotylus Argonautae. 



In this way we readily account for the singular fact, that a 

 colourless layer is constantly found upon the exterior of the 

 dorsal crest, while the layer of chromatophora lies internally 

 upon the so-called capsule of the testis. 



The membrane of the sac then belongs to the future Hectoco- 

 tylus. This was seen most clearly in that specimen in which, 

 as has been already noticed, the sac opened spontaneously ; for 

 upon touching the Hectocotylus frequently it detached itself from 

 its delicate pedicle so as to carry away the inverted sac with it. 



