146 VERANY AND VOGT ON THE IIECTOCOTYLI 



dies which run along the two sides of the flabellum, and whirh 

 may be followed even beyond the piston-like enlargement. These 

 two cutaneous muscles are accompanied through their whole 

 length by two venous trunks which send off numerous ramifica- 

 tions, forming a capillary network over the whole surface of the 

 flabellum. These ramifications are especially remarkable at the 

 very extremity of the flabellum, where they are perceived very 

 easily, the tissues being at this place completely transparent. 



The extreme mobility of the flabellum perhaps plays a part in 

 the physiological functions of the hectocotyliform arm. We 

 have seen this appendage continually in movement, as if it were 

 feeling about to fix itself somewhere. It embraced the arms, 

 and even the body of the animal to which it belonged, but it 

 disentangled its coils again without its being possible to discover 

 to what end all these motions tended. We shall bring forward 

 by-and-by an observation of M. Kolliker's, which will, perhaps, 

 set other observers upon the track of the physiological function 

 of this part. 



We have yet to mention a last peculiarity of the structure of 

 the hectocotyliform arm; it is the existence of a sac of con- 

 siderable extent, which is visible upon the posterior or dorsal 

 face of the arm near its base, and therefore opposite to the ace- 

 tabula. This sac (PI. I. fig. 3 a) is 15-20 miUimetres in length, 

 and is elongated, inasmuch as it lies along the dorsal face of the 

 muscular cylinder of the arm. It is distinguished very easily 

 throughout its entire extent by its deep colour, whilst the rest 

 of the hectocotyliform arm is wholly colourless. This colora- 

 tion is due to the chromatophora which beset the whole internal 

 surface of the sac, and may be seen shining through it. A semi- 

 lunar aperture, situated upon the dorsal face of the arm imme- 

 diately above the pedicle, leads into this sac, which is a true cul- 

 de-sac, and is closed upon all sides. We have never found any- 

 thing in this sac, which is obviously a diverticulum, an involu- 

 tion of the skin of the body itself, and whose formation depends, 

 as we shall soon see, upon the mode of development of the arm. 



We have already many times mentioned the oval vesicle car- 

 ried upon a delicate pedicle, which was observed in many indi- 

 viduals in the place of the hectocotyliform arm. Upon closely 

 examining this vesicle, we see that it is lined in its whole extent 



