AND THE MALES OF CERTAIN CEPHALOPODS. 145 



a little fissure which leads into the cavity of this sac, and by 

 which the animal is enabled to evolve the spiral filament hidden 

 in its interior. We have frequently seen instances of this oc- 

 currence ; the filament passed by its pointed extremity out of 

 this cleft and slowly unrolled itself; the sac all the while being 

 agitated by repeated contractions which aided the expulsory 

 movement. The filament itself performs very marked vermi- 

 cular movements, which we can only compare to the motions of 

 the tentacular filaments of certain tubicolar worms, particularly 

 qf the Terebellce. The little sac is wholly contracted w^hen the 

 thread has passed out of it, and it then exhibits under the mi- 

 croscope (PI. IV. fig. 6 e^f) very numerous rugosities, presenting 

 a sort of very pretty watered pattern. It evidently consists of 

 two membranes, one external, which is nothing more or less than 

 the continuation of the skin w^hich covers the whole arm, and an 

 internal muscular layer which is continued by two muscular 

 bundles upon the two sides of the flabellum. This sac con- 

 tracted upon the flabellum, which it had contained, has been 

 also seen by the authors who have written upon the Hectoco- 

 tylus of the Argonaut', and lastly by M. Kolliker, who 

 regards it as a membranous appendage without any other sig- 

 nification. 



The flabellum itself (PI. IV. fig. 8) is composed especially, as 

 we have said above, of the muscular cylinder of the arm, whic h 

 occupies its centre without interruption as far as its extremity, 

 gradually becoming smaller. This cylinder ends by a point at 

 the very extremity of the flabellum, which is at first completely 

 rounded, but becomes flattened little by little like a lance-head 

 towards its end. At the distance of a few milUmetres from the 

 extremity of the flabellum the muscular cyhnder becomes sud- 

 denly' thickened, and takes the form of a piston (PI. IV. fig. 8 b) ; 

 its internal cavity even is here widened. At this same place we 

 have always seen a considerable mass of blood-corpuscles dis- 

 tending the enlarged cavity. The median vessel which occu- 

 pies the centre of the muscular tube, ends therefore here in a 

 kind of reservoir, whilst the muscular axis continues as a hollow 

 cylinder as far as the extremity. The external membrane which 

 invests the flabellum is, especially at its base, very loose, and 

 exhibits proper undulatory movements due to two muscular bun- 



SCIEN. MEM.— A^a/. Hist. Vol. I. Part II. 10 



