710 DE. W. B. CAEPENTER ON THE STRUCTUEE, PHYSIOLOGY, AND 



interposed a ligamentous (not muscular) substance ; this is seen in the basal joints to 

 be as thick on the oral side as it is on the aboral ; but as we advance towards the middle 

 of the cirrhus, the thickness of the interarticular substance is seen to be much greater on 

 the aboral side, the form of the segments being so modified as to admit of considerable 

 flexure in that direction, whereby the prehensile power of the claw is much increased. 

 When a cirrhus is subjected to decalcification, it is found that this interarticular sub- 

 stance corresponds with the general sarcodic basis of the skeleton in every particular, 

 save in the great abundance and regular distribution of its fibrous component. The 

 fibres can be distinctly traced passing straight and direct between the articular surfaces 

 (Plate XLIIT. fig. 5), not terminating there, however, as do the muscular fibres (§ 19), 

 but becoming incorporated with the basis-substance of the calcified segments. And it 

 seems clear from the history of the development of these organs, that the basis-sub- 

 stance of the calcified segments and the interarticular substance which connects them, 

 are originally similar ; but that whilst the former is altered by the deposit of its calca- 

 reous reticulation, the latter is changed by the augmentation of its fibrous component. 

 The connexion of the basal segment with the socket of the centro-dorsal plate to which 

 it is articulated, is effected by precisely the same kind of ligamentous substance (^ 24). 



28. When the segments are separated by boiling in a solution of potass, and their 

 opposed faces are examined (Plate XXXIII. fig. 8), it is seen that each is perforated by 

 an axial canal of about one-fifth of its diameter, around which is a projecting articular 

 surface; and that on the oral and aboral sides of this projection there are t^'^'o depres- 

 sions for the lodgment of the interarticular ligament. In the basal segments (a), the 

 canal with its surrounding projection is central, and the oral and aboral depressions are 

 of equal size, or nearly so ; but in the terminal segments (b) the canal and articular sur- 

 face are nearer the oral side, and the ligamental depression is both larger and deeper on 

 the aboral. This difference is still better seen in longitudinal sections of these two por- 

 tions of the cirrhus respectively (Plate XXXV. figs. 2,3); in which also we observe that 

 the terminal claw is attached to the last segment at a, a, by a plane surface admitting of 

 no movement, the two being held together by continuity of their animal basis-substance. 

 When a thin portion of such a section is examined with a sufficient magnifying power, 

 the calcareous reticulation is seen, as in other cases (^ 24), to be much closer at the 

 articular surfaces than it is elsewhere ; and in the portion of the last segment which is 

 prolonged into the opposing process (fig. 3), the reticulation is covered with a layer of 

 homogeneoiis shell-substance like that which forms the solid pillars in the spines of the 

 EciuxiDA. Of this substance nearly the whole solid shell of the terminal claw is com- 

 posed, its interior, however, being occupied by a large cavity which is continuous with 

 the axial canal (c, c) of the cirrhus, and the imier layer of its wall having the characteristic 

 reticular structure. 



29. The Axial Canal which thus runs from the base to the apex of each Cirrhus, is 

 continuous at its base with the canal which passes from the concavity of the Centro- 

 dorsal plate to the summit of each articular tubercle (^ 24). It was supposed by Professor 



