THE CRINOIDEA 



105 



radial. Nerves from these cords are given off to the stroma of 

 the cup-plates. This system, as experimentally proved, chiefly 

 by \V. B. Carpenter (1876 and 1884), in opposition to the 

 scientific opinion of his time, is a senso- motor nerve -system, 

 governed from the nervous capsule of the chambered organ. 

 By means of the commissures the motion of all muscles is cor- 

 related. Primitively the cords lie on the inner surface of the 

 cup; they then become bordered by ridges of stereom, and 

 finally enclosed within the cup-walls. Branches from these nerves 

 unite with the interradial nerves that proceed from the circum- 

 oesophageal nerve-ring. 



To turn to the Stem. We have already traced its probable 

 origin as an evagination of the many-plated theca of Amphoridea, 

 and the gradual introduction of order into the irregular plates (p. 4 8). 

 In the pentamerous Crinoidea, these naturally became subjected to 

 pentamerism ; and evidence of many of the older crinoids shows that 

 the plates were at first hexagonal and arranged in alternating circlets 



FIG. XIII. 



Evolution of Pentamerism in the 

 Stein. IJointsurface of a columnal com- 

 posed of five sections, which alternate 

 with the angles of the stem-lumen ; 2, 

 portion of a stem composed of hexag- 

 onal alternating plates, which in 3 be- 

 come arranged more definitely in hori- 

 zontal rows ; 4, continuance of the 

 process results in columnals of five pen- 

 tameres. The figures are of Botryo- 

 crinus stems (after Bather). 



of 5, just as plates of the theca (Fig. XIII.). The next stage was 

 that in which the plates no longer alternated, but were arranged in 

 horizontal rows divided by five longitudinal sutures. Finally, the 

 pentameres of each row became fused to form a " columnal," still 

 pierced by a wide lumen. This regularity was perhaps connected 

 with the extension into the lumen of a vessel from each of the five 

 lobes of the chambered organ, with its nerve-sheath (axial cord) ; 

 the five cords surrounded a prolongation of the axial organ. In a 

 monocyclic crinoid the axial cords would be interradial, as are the 

 lobes of the chambered organ, while the pentameres would alternate 

 with the basals and be radial. In a dicyclic crinoid the cords would 

 be radial, the pentameres interradial. The exterior angles of the 

 stem usually correspond with the pentameres, but not always. 

 The cirri, or side-arms of the stem, correspond, for reasons that 

 will appear presently, with the axial cords. The lumen of the 

 stem is often split up into grooves by ingrowths of stereom ; and 

 these grooves primarily contain the axial cords, and may even form 

 closed canals containing the cords, but this correspondence is not 

 inevitable. The so-called "law of Wachsmuth & Springer," 



