CLARK: NERVOUS SYSTEM OF CRINOIDS 67 



and the derivatives from the primary interradial nerve cords do 

 not join radially, though they approach each other. The intra- 

 radial commissures do not extend across the radial gap between 

 the nerve branches, but remain as five separate intraradial com- 

 missures joining the secondary nerve cords, just like the intra- 

 basal commissures in the pentacrinites. 



The dorsal nervous system of the crinoids is therefore in effect 

 composed of five interradial nerves and their derivatives, each of 

 these nerves being comparable to the single double ventral nerve 

 cord of the arthropod or of the annelid. 



Whenever the arm of a crinoid branches, forming an axillary, 

 a curious chiasma occurs within that axillary. The entering nerve 

 at once divides, the two derivatives emerging by the two central 

 canals of the two distal articular faces. Just before they emerge 

 these two branches are connected by a transverse commissure. 

 Oblique commissures run from a point just beyond the division 

 of the main cord to the transverse commissure, crossing each 

 other distally. 



This chiasma appears to be a repetition of the nerve branching 

 within the calyx; the two oblique commissures represent the inner 

 branches of the two primitive nerve cords, now fused into one, 

 which have become atrophied through disuse; the transverse com- 

 missure represents the intrabasal commissure of the calyx, and 

 is not developed unless the intrabasal commissure is present, 

 being quite independent of the presence or absence of the intra- 

 radial commissure. 



The supposition that the chiasma within the axillaries is in 

 reality a reduplication of the conditions found within the calyx 

 rather than an original structure peculiar to and developed within 

 the axillaries is strongly supported by the general agreement to 

 be found between the radials and the axillaries. In interpolated 

 division (cf. Proceedings U. S. National Museum, vol. 35, p. 113) 

 of the post-radial series or arms, such as is seen for instance in 

 most of the recent comatulids and in the pentacrinite genus 

 Endoxocrinus, each axillary supports a pair of ossicles which are 

 the exact equivalent of the first two ossicles beyond each of the 

 other axillaries, and of the first two ossicles beyond the radials. 



