352 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



five entirely distinct connectives, in every way comparable to the five isolated intra- 

 basal commissures. 



Indeed in Encrinus liliiformis (as worked out by Beyrich) the truth of this is 

 well brought out, for the diverging branches from the primary interradial nerve cord 

 do not meet, but remain always at a considerable distance from each other, so that 

 the five commissures connecting the branches are widely separated. Encrinus is 

 a genus of the palaeozoic type with biserial arms, and therefore is much more primi- 

 tive (in its arm structure at least) than the recent uniserial types. Its brachial 

 nerve cords thus may be confidently assumed to be also more primitive, and to indi- 

 cate the course by which the nerves of the recent comatuhds and of the pentacrinites 

 have attained their present complexity. 



In Apiocrinus parkinsoni the course of the canals has been worked out, and it 

 is found that the derivatives from each of the primary interradial nerve trunks 

 always keep separate, running parallel through the IBr^ diverging in the IBr, 

 (axillary) which has no chiasma, and entering the arms, the two arms of each pair 

 being innervated from the adjacent interradial areas and entirely independent of 

 each other. A commissure connects the diverging branches of each primary inter- 

 radial nerve trunk within the radials, but there is no proximal (intrabasal) com- 

 missure. 



The clue to this interpretation of the nervous system of the crinoids is furnished 

 by the axillaries; within each axillary we find a complicated chiasma (fig. 62, p. 89) ; 

 the entering nerve branches at once, the two derivatives emerging at the center of 

 the two distal articular faces; a commissure connects these two derivatives just 

 before they emerge; just beyond the division of the original nerve cord an oblique 

 commissure is given off to the transverse commissure, the two oblique commissures 

 crossing at their distal ends. 



Close examination shows that the division within the axillary is exactly the 

 same as the division of the primary nerve cords within the basals and the radials. 

 The axillary is composed primarily of two fused ossicles, as is shown by the articu- 

 lations by which it is joined to the preceding and succeeding ossicles; the significance 

 of these will be fully explained later. 



In Encrinus each of the two nerves which enter the axillary branches, the inner 

 derivative crossing over to the opposite side, and from each of the two distal faces 

 of the axillary two nerves are given off side by side, one of each of the pairs being 

 from the left hand and the other from the right hand large nerve which entered the 

 axillary. Thus in Encrinus the interradial nerves do not intermingle, but run side 

 by side, not fusing to produce a radial nerve cord, as is the case in the comatuhds and 

 in the pentacrinites. Encrinus possesses the intraradial commissures, but not the 

 intrabasal ; and it has no transverse commissures in the axillaries. But hi the pen- 

 tacrinites there is an intrabasal commissure, and there is also a similar commissure 

 within the axillaries. 



The chiasma within the axillaries of the pentacrinites and of the comatuhds 

 therefore is a reduplication of the conditions seen in the primitive nerve cord; the 

 the small diagonal fibers represent the original branching of the two primitive nerves, 

 though as a result of the fusion of these two nerves into one they have become prac- 



