ARGOXAUTA ARGO AND THE HECTOCOTYLI. 77 



line. The smallest were not much shorter, and some were 

 longer. The breadth in the middle of the villi was generally 

 0-15, hardly under 0'12, but as much as 0'22 of a line. These 

 were Hectocotyli with a freely projecting penis. 



In two others, on the other hand, whose penis was hidden, 

 the length of the villi was but rarely over 0*6-0*7 of a line, and 

 the most were shorter. The breadth was at the base rarely more 

 than 0'12 of a line, and diminished rapidly to 0*06 or 0'04 of a 

 line. The villi namely, had here the form of a rapidly dimi- 

 nishing and pointed cone, whilst in specimens with well- deve- 

 loped gills the diameter of the gills in the outer half surpassed 

 at times that of the base, and the end was more rounded than 

 pointed. 



Since it may be assumed that Hectocotyli with larger gills have 

 in general progressed further in their development than those 

 with smaller ones, we have here a further evidence that the penis 

 rolled up under the skin of the latter is a younger stage of de- 

 velopment than the common form of the free penis. 



The Hectocotylus of the Tremoctopus, then, is very strongly 

 distinguished from the two others by the want of a pigmented 

 dorsal capsule ; by the position of the seminal coil in the cap- 

 sule at the end of the body, and the peculiar structure of the 

 ductus deferens ; lastly, by the presence of gills : and, indeed, 

 from the great discrepancies which exist among the Hecto- 

 cotylus-bearing males of Cephalopods among nearly allied spe- 

 cies, we should be prepared for thorough differences among 

 themselves. 



Upon the other hand, the analogy of the Hectocotylus of the 

 Tremoctopus with the others in all essential points is so great, 

 that although there is a complete want of all direct observations, 

 we must assume it to have a similar origin, and that one day an 

 entire male Tremoctopus belonging to this Hectocotylus will be 

 discovered, the exact investigation of which will doubtless be 

 still more interesting than in the Argonaut. However that may 

 be, all three Hectocotyli must be kept in mind in attempting to 

 determine the nature of the Hectocotyli in general. 



