288 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Any movement of the kind just described would first affect the outer border 

 of the side plates, thence gradually extending itself downward; and we find that 

 the supplementary plates become separated off, though a rapid deepening of the 

 notch between the distal outer process and the main body of the plate in which 

 the tip of the tentacle ordinarily lies, accompanied by a progressive eversion of 

 the plate. 



In Oceanometra gigantea the supplementary plates undoubtedly represent the 

 inner portions of the curved ridge on the inner side of the distal borders of the 

 side plates which have become detached from the main body of the plates through 

 the shifting proximally of the sacculus and the moving downward of the tentacle tip 

 when the covering plates are closed over the ambulacral groove, these changes 

 having resulted in the resorption, or nonappearance, of any portion except the 

 crests of the two sections of the distal ridge. 



Although the adambulacral deposits in the comatulids are sporadic in origin, 

 in the group as a whole as well as in many of its subdivisions, and of purely 

 secondary significance, without any such direct homologies as we are, for instance, 

 able to trace in the primary plates, still there are certain possibilities to be con- 

 sidered in connection with them. 



It has already been explained that the crinoid arm is not an appendage, prop- 

 erly speaking, but an enormously elongated branched eversion of the body wall 

 from a point exactly on the line where the heavily calcified dorsal surface joins 

 the perisomic surface, which carries out within it extensions from all the ring 

 formed structures about the mouth and gullet, as well as an extension, the dorsal 

 nerve cord, from the central nerve mass. The only organs not continued outward 

 into the arms are (1) the digestive tube, (2) the axial organ, which arises from 

 the wall of the digestive tube and is probably the homologue of the so-called 

 notochord in other invertebrates, and (3) the chambered organ, which is intimately 

 connected with the axial organ and lies within the central nervous system. 



Within the arm the extensions from the ring systems about the mouth and 

 gullet maintain their original relationships ; and as in the calyx the central dorsal 

 nervous mass is isolated in the centrodorsal, so in the arms its extension is isolated 

 within the calcareous substance of the brachials and pinnulars. 



Before the formation of the arms we find in the young crinoid five lappets 

 about the mouth, over which they can be closed, each of which has associated 

 with it three tentacles, and at the base on either side a sacculus. 



Along the ventral side of the arms and pinnules, bordering the ambulacral 

 groove, just as the lappets about the mouth border upon the peristome, with the 

 same relationships with the extensions of the ring systems as the oral lappets 

 have to the ring systems themselves, and each with a group of three tentacles and 

 a sacculus on either side of the base, we find a continuous series of lappets. 



From their size, structure, and relationships we appear to be justified in 

 assuming that the lappets bordering the ambulacral grooves of the arms and pin- 

 nules are not only strictly homologous with those about the mouth, but with the 

 latter at the stage at which the arms are first formed; that is to say that while 

 the lappets about the mouth undergo many changes in passing to the adult stage. 



