25G 



BULLETIN OF THE 



canals, of which the latter has been more studied and is known to carry a 

 nerve and a water-tube. The outer surface of the arm-bone (Fig. B) pre- 

 sents, above, a hollow (4) in which rests the umbo (1) of the next piece be- 

 yond it; below are two depressions (5) into which fit the knobs (2), and 

 between which is a peg (G) fitting in the socket (3). On each side of the lower 

 edge is a triangular swelling (/•), which is the outer wall of the tentacle-socket. 

 Seen from above (Fig. C), the upper longitudinal canal (l') divides the piece 

 in two, leaving on either side an elongated triangular surface on which rest 

 the upper arm-plates. On the outer side may be seen the upper surface of 

 the hollow (4) and the articulating peg below it (6) ; and within is the upper 

 surface of the articulating umbo (1). A view from below (Fig. D) shows the 

 lower longitudinal canal (/) ; then without is the articulating peg (0) and the 

 two sockets of the tentacles (r) ; within are the great lower muscle-fields («■), 

 the two articulating knobs (2), and the socket (3) for the articulating p< :g. 

 (For detailed views see PL VII., Figs. 7-10.) From the way in which the 



joint is held by the umbo above and the peg below, a vertical motion of the 

 arm upward is difficult where these parts are well developed, while the 

 lateral motion is comparatively a free one. As a fact, the chief motion in 

 the living is a lateral one, and only certain species roll their arms in a verti- 

 cal plane, and this rolling is downward and not upward; the umbo must then 

 slip outward and downward, while the peg must press deeper in its socket 

 (Fig. G, lettered like the others). 



A great modification is to be seen in the bones near the tip of the arm, 

 which are much elongated, and are quite different in detail of structure. 

 Along the upper surface runs a very wide and deep longitudinal canal 

 (Fig. E, t') ; from the outer end projects a forked process, which is the ar- 

 ticulating peg (6) ; at the inner end may be recognized the articulating knobs 

 (2), and the socket (3). On the lower surface may be seen the same parts 

 (Fig. F) and the corresponding canal (/), which is slightly marked. (See 

 also Fl. TIL, Figs. 1G, 17). 



