104 Prof. J. Steenstrup on Hectocotylua-formation 



large and spacious cutaneous sac, which lies deeper than the 

 root of the arm, and occupies essentially the same place as the 

 large folds in the genera Sepia, Rossia, &c., into which the ten- 

 tacles may be more or less retracted and received. Figure 7, 

 which represents a male specimen of this species three times 

 the natural size, shows at once that the male has only seven 

 arms, which are all regularly developed, and that the absent 

 arm is actually the right arm of the third pair. On closer ex- 

 amination, however, it is seen that the place where this right 

 arm should be situated is as it wei'e swelled up, and that by 

 this swelling both the fourth pair of arms and the funnel are 

 not a little displaced towards the left side. Under a lens it is 

 easy to perceive that the cause of this displacement is a very 

 long, rolled-up arm, which occupies the space between the fun- 

 nel, the eye, and the root of the arm, and which is covered only 

 by a skin so thin and transpai-ent, that the eye may easily trace 

 the curves of the arm, and distinguish their finely fringed mar- 

 gins and the individual acetabula which are turned towards the 

 skin, which, however, in consequence of the mode in which the 

 ai-m is rolled up, are but very few. In the enveloping mem- 

 brane I have been unable with the lens to detect either an 

 actual opening through which the arm might issue forth, or a 

 line or impression, to indicate where a division might sub- 

 sequently occur, as is known to be the case in T. Carencp ; but 

 perhaps this would have been more distinct on the fresh speci- 

 men. After opening the cutaneous sac by an incision with a 

 sharp knife, it was easy to draw forth the remarkable Hectoco- 

 tylus-avm. This is represented, when thus extracted, in fig. 8. 

 It was quite colourless throughout its whole extent, like the 

 previously observed Hectocotyli of Argonauta and Tremoctopus, 

 but individual chromatophora existed in the membrane covering 

 it. Its length is several times that of the corresponding arm on 

 the opposite side of the animal, although this is longer than the 

 very long first and second pairs (by which, indeed, this species 

 is characterized), and bears no less than 33 pairs of acetabula; 

 therefore a greater number than is supported by any of the 

 other arms. These acetabula are of nearly equal size through- 

 out the entire length of the arm, and the arm itself is also 

 nearly uniform in breadth throughout. At the apex the ami 

 swells into a nearly pyriform part, which is destitute of suckers ; 

 and along one side of this swelling there is a slight furrow and 

 fold of skin, of which the latter appears to be traceable down 

 the arm for a short distance. At the base of the swelling and 

 close to the extreme sucker of the arm, is the origin of a long 

 lash or filament of 55 mill, in length, the basal portion of which 

 appears as if surrounded by a thin sheath. It is still to be re- 



