724 DE. W. B. CARPENTER ON THE STRUCTURE, PHYSIOLOGY, AND 



segment that bears it by ligaments only, is much shorter than that which follows it 

 (Plate XXXVII. fig. 3) ; and the length of the succeeding segments presents a consi- 

 derable uniformity, until we approach the termination of the pinnule, where we often 

 find a set of segments much shorter than the rest (Plate XXXVIII. fig. 3), suggesting 

 the idea that these are the last formed. The number of segments in the pinnules is 

 found by no means constant, when we compare either different pinnules of the same 

 Arm, or corresponding pinnules in different individuals. Thus the first, which from 

 their peculiar relation to the mouth I distinguish as the oral pinnules, very commonly 

 have about thirty-five segments ; but I have not unfrequently counted as many as forty, 

 and in one instance forty-five. And while the ordinary pinnules of the hasal part of 

 the Ai'm have usually about twenty segments, the number not unfrequently rises to 

 twenty-five. In the middle part of the Arm (Plate XXXVII. fig. 1) the number of 

 segments in the pinnules may average eighteen ; and in its terminal portion we find 

 this ra^jidly diminishing from sixteen to half that number (Plate XXIXVIII. fig. 4). 

 The general conformation of the individual segments closely corresponds with that of 

 the segments of the dorsal cirrhi (§ 28). They are nearly cylindrical, but somewhat 

 compressed on their ventral aspect ; and each of them (at least in the pinnules of 

 the basal and middle parts of the Arms of well-developed specimens) has the dorsal 

 margin of its distal end fringed by a set of short oblique spines (Plate XXXVII. fig. 3). 

 The articular faces of the segments are formed on nearly the same plan with those of 

 the segments of the dorsal cirrhi (§28), the opening of the central canal by which every 

 segment is traversed, being surrounded by a slightly elevated ring, sometimes extended 

 into a transverse ridge, and a depression being left by the bevelling-away of the surface 

 in both directions, that serves for the lodgment of interarticular ligaments. But besides 

 these depressions, there is in each of the basal segments, at least of well-developed 

 pinnules, a small but deep notch in the ventral margin of each articular surface, but 

 deeper in the distal ; and this lodges a minute muscle (Plate XXXVIII. fig. 10), by the 

 action of which the pinnules can be so flexed that those of the two sides of the Arm 

 are brought towards each other, and also towards the line of its axis, — the converse 

 movement of extension being efi"ected (as in the Arms) by the elastic ligaments, when 

 the muscles are relaxed. 



Repaeations. 

 55. It is well known to Naturalists that a remarkable degree of reparative power is 

 exhibited by Echinoderjuta generally; and owx Antedo7i, so far from constituting an ex- 

 ception, afibrds abundant exemplifications of its operation. For when we have the oppor- 

 tunity of examining a large number of specimens brought up together by the di'edge, we 

 are sure to meet with several which have obviously sustained losses either of entire Arms 

 or of portions of Arms, and in which the lost parts are being reproduced on a smaller 

 scale. We so often find that the fracture has taken place at a syzygij (Plate XXXVIII. 

 fig. 6) that the question suggests itself whether there is any special reparative power at 



