DE\^LOPMENT OF ANTEDON (COMATTJLA, LAMK.) BOSACETJS. 723 



length is but little greater than that of an ordinary single segment. — The same general 

 proportions are maintained through a considerable part of the length of the Arm, the 

 tendency being, however, to a further reduction in the diameter of the segments, without 

 a corresponding reduction of their length ; so that notwithstanding the increased obli- 

 quity of the articular surfaces, the muscular fossae of the proximal and distal faces of 

 each segment, instead of being separated merely by the thin vertical lamellae which 

 form the floors of both, lie as it were along its ventral surface, and yet do not come 

 into close approximation. 



53. Terminal Portion. — Passing onwards to the termination of the Arms, we find a 

 progressive diminution in the diameter of the segments, without a corresponding reduc- 

 tion in their length ; so that their proportions now differ widely from those of the basal 

 segments (Plate XXXVIII. fig. 4). We still observe, however, an alternating obliquity 

 in the disposition of their articular surfaces as seen from the dorsal side ; a sort of notch 

 being left, first on one side and then on the other, which is occupied by the elastic 

 ligament. On separating the articular surfaces, we no longer find them presenting the 

 characteristic ligamental and muscular fossae ; but there are nearly plane surfaces above 

 and beneath the central aperture, to which the ligaments are attached, whilst the muscles 

 lie in elongated fossae excavated in the ventral face of the segments (c, c, fig. 2, A, b). The 

 articular sockets for the pinnae (p, p) show themselves on the lateral surfaces of the seg- 

 ments at about the middle of their length, and are quite removed from the muscular fossae. 

 — Of the normal mode of termination of the Arms, I am unable — after an exami- 

 nation of some hundreds of specimens — to speak with certainty ; for I have never met 

 with a specimen of this animal possessing in other respects the characters of maturity, 

 which presented such a gradational diminution in the dimensions of the terminal seg- 

 ments as might be fairly expected from the general plan of its structure. In a large 

 proportion of cases the Arms end so abruptly as evidently to show that their terminal 

 segments have been broken off. And when they do present such a termination as is 

 shown in Plate XXXVIII. fig. 4, this has all the characters of immaturity (§ 67) ; and 

 must be regarded as marking, not the completion of the growth of the Arm, but either 

 the continuance of its normal extension, or the reproduction of the portion which has 

 been lost. 



54. Pinnce. — The pinnules with which the Arms are fringed (Plates XXXVII. figs. 

 1, 2, 3, XXXVIII. figs. 4, 10, 11) are composed of articulated segments nearly cylin- 

 drical in form, and gradually tapering from their base to their extremity, which bears a 

 peculiar segment furnished with five or six minute hooks (Plate XXXVIII. fig. 3). 

 The size and length of the pinnules vary considerably in different parts of the Arm, 

 those near the base being not only longer, but also stouter in proportion to their length. 

 The first pair of pinnules, which are attached to the Second Brachial segments, are 

 much longer than those which succeed, often in fact attaining double their length ; and 

 these, as already mentioned, are nearly always seen in the living state bending over the 

 ventral disk (Plate XXXI. fig. a). The first segment, which is articulated to the Brachial 



