MORPHOLOGY 37 



Perforation of the radials or brachials for the passage of the axial nerve 

 cords, such as is common in the Recent and many Mesozoic crinoids, and is 

 found in some Inadunata, has not been observed in this group, except in some 

 of the brachials of Forbesiocrinus and perhaps in Synerocrinus. Indication of 

 such perforation in the former is well shown on Plate XXIV, figures 8-15, 

 which seem to represent stages of its development from a ventral furrow. In 

 those parts of the calyx below the brachials the course of the nerve cords can 

 in many cases be distinctly traced externally by ridges representing folds in the 

 plates which pass from one to another. Such folds are very plain in Pycnosac- 

 cus, Hormocrinus, Lecanocrinus waukoma, Steganocrinus pentagonus, and in 

 Stephanocrinus angulatus, but they have usually been considered as mere sur- 

 face ornament. Bather 1 and Wilson 2 have noted the occurrence of a ridge 

 upon the anal series in certain Camerata as showing the presence of a nerve 

 cord passing beneath it to control the tube. There seems no reason to doubt 

 that the position of the principal nerve branches supplying other parts of the 

 crown is in some cases indicated by similar ridges upon the exterior of other 

 plates. Such ridges occur in only a few genera of the Flexibilia, but their office 

 as indices of the course of the nerve cords is very clear. That external evi- 

 dence of the course of the nerve branches is not more generally afforded might 

 be expected from the more diversified character of the nervous system in the 

 ancient crinoids, whose highly specialized calyx is in marked contrast to the 

 very simple and greatly reduced calyx of the modern forms. In some genera 

 and species the ridges are broken up into bars, become spinose or nodose, or 

 are reduplicated by systems of parallel ridges which obscure the primary ones. 

 In many species the primary ridges are entirely absent, while in others they 

 may be easily determined. 



In normal radial innervation in dicyclic crinoids the central nervous organ 

 rests within the inf rabasals and its lobes are radial in position. From each lobe 

 two obliquely diverging nerve cords pass to the centers of the basals, where 

 they unite and are connected by the horizontal cord known as the basal com- 

 missure. From the center of the basals the nerves again diverge and pass to 

 the radials, where after uniting in the radial commissure they continue on into 

 the arms (text-fig. 3). In monocyclic crinoids radial innervation is even 

 simpler; the nervous organ lies within the basal ring and its lobes are inter- 

 radial in position. The two nerve cords from each lobe pass up into the radials, 

 where they unite in the radial commissure and then go into the arms (text- 

 fig. 5). Innervation of interradial structures in either form is probably effected 

 by secondary branches from the radial trunks or from those arising on the 

 basals. 



1 In Lankester's Zoology, pt. 3, 1900, p. 119. 



2 Journal of Geology, vol. 24, 1916, p. 550. 



