l6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 'J2 



radial (mid-somatic) in position, in consisting of single cords in- 

 stead of paired cords with commissures, and in lacking a well de- 

 veloped central organ. From its lack of any relation to the preoral 

 lobe and its general indefiniteness of structure it is difficult to see how 

 it can be in any way homologous with the apical nervous system 

 of the crinoids. It is probably a special feature peculiar to asteroids. 



The apical nervous system of the crinoids, from which the whole 

 animal with the exception of the peristomal region and its extensions 

 is innervated, is absent in the asteroids, or rather it has become merged 

 into the circumoral structures and their derivatives. 



It has been shown that the crinoids before the development of the 

 arms are encased in plates developed over the somatic divisions, but 

 that beginning with the appearance of the arms the development be- 

 comes wholly intersomatic or radial. In the asteroids the develop- 

 ment is radial from the first appearance of the plates, the interradial 

 (mid-somatic) body covering seen in the young crinoids not appear- 

 ing at all. 



In the crinoids there are at first no plates belonging to the ventral 

 surface, the basals being dorsal and the orals ventrolateral, but the 

 latter become ventral plates after the formation of the arms, which 

 themselves are composed of a series of dorsal ossicles carrying exten- 

 sions from the ventral structures on their ventral surface. In the 

 asteroids the plates at their first appearance represent those of the 

 crinoids after the formation of the arms as far as their bifurcation, 

 minus the orals. There is a central plate, corresponding to the infra- 

 basals in the crinoids ; about this are five basals, corresponding to the 

 five basals of the crinoids ; beyond and alternating with these are five 

 terminals, corresponding to the radials of the crinoids, but always 

 single and showing no indications of a primarily paired condition as the 

 crinoid radials do. On the opposite (ventral) surface are five pairs of 

 plates, one pair in each radial division, corresponding to the first two 

 post-radial plates in the crinoids, but side by side instead of tandem. 



Thus whereas the extensions of the crinoid body — the arms — are 

 just on the border between the dorsal and ventral surfaces and are 

 composed dorsally of dorsal ossicles and ventrally of extensions of 

 ventral structures, in the asteroids the dorsoventral edge of the body 

 has moved dorsally so that the dividing line between the dorsal and 

 ventral surfaces falls between the radials and the succeeding plates, 

 and the whole ventral surface is encased in plates which are repre- 

 sented in the crinoids on the dorsal surface only. 



The asteroids therefore differ from the crinoids in the temporary 

 attachment of the larvce; in the relatively slight alteration in the posi- 



