82 Prof. J. Steenstrup on Hectocotylus-/orm«/ion 



length on this side, that it is impossible to doubt that the arm 

 is thereby adapted to some particular purpose, of which we can- 

 not suppose that it is of subordinate importance to the animal, 

 because its transformation occurs in so great a number of species 

 of the class, and bears its peculiar characters in each natural 

 genus. 



When the metamorphosis is traced from form to form, we 

 may see distinctly, in my opinion, that the arm with its peculiar 

 structure enters entirely or partially into the service of repro- 

 duction, and in the first case even becomes wholly unfitted for 

 the part which it otherwise has to play, namely to act as an 

 organ of motion (swimming or creeping) or in the prehension of 

 food. This metamorphosed arm consequently betrays its close 

 alliance with the Hectocotylus-formation in the two Octopod 

 genera Argonauta and Tretnoctopus, Delle Chiaje, as the recent 

 investigations of Filippi, Verany, Vogt, and H. Miiller have 

 placed it beyond a doubt that the Hectocotylus or parasitic crea- 

 ture found so frequently upon female Argonauts and sometimes 

 upon female Tremoctopodes, — which was first regarded, in accord- 

 ance with the views of Delle Chiaje, as a parasitic animal or 

 Entozoon, but subsequently, in accordance with the sagacious 

 combinations of Kolliker, as a metamorphosed male Cephalopod 

 destined to lead a parasitic life upon its female, — is by no means 

 a complete organism, but only an arm of the male Cephalopod, 

 which, being filled with semen, separates from it and adheres to 

 the female for the purpose of fecundation. In order to under- 

 stand this Hectocotylus-formation, which during the last three or 

 four years has attracted so much of the attention of naturalists, 

 more correctly, and in its connexions, the observations which I 

 here bring forward will furnish an important key ; but with the 

 interest which they possess in this respect, they also unite, as it 

 seems to me, no small importance in a systematic point of view. 

 The peculiarity here referred to furnishes an additional hint for 

 the comprehension of what does and what does not belong to 

 this class, and in many cases it gives good specific characters 

 for the distinction of nearly aJlied species, without considering 

 the value which it possesses as an external [sexual] character in 

 individuals of the same species, especially as such characters have 

 hitherto been missed ; and these are at the same time so readily 

 intelligible and so striking to the eye, that one can hardly help 

 wondering that they have not previously been observed and 

 brought into use. 



After these few introductory words I pass immediately to the 

 description of the most essential difierences of form in this me- 

 tamorphosis represented in the figures upon the accompanying 

 plates, merely remarking, preliminarily, that in the order in 



