154 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



position at the base of the anterior postradiai series, which it always occupies in 

 the young, far to the right, so that it comes to lie midway between the bases of the 

 anterior and the right anterior postradiai scries (figs. 25-28, p. 69). 



The various ring s3-stcms mamtain their original position about the mouth; 

 hence the left posterior ray, orienting from the position of the mouth and the central 

 anal tube, has now become posterior, and is thereby placed at a great disadvantage 

 through bemg at a greater distance from the circumoral ring systems than any other 

 ray, and typically it becomes atrophied, entirely losing its tentacles, ambulacral 

 grooves and ambulacral nerves (fig. 27, p. 69). This condition is often found also 

 on the left anterior and right posterior rays, now become the left and right latero- 

 posterior, these being at a considerable disadvantage -when compared with the two 

 anterior rays, one of which is situated on either side of the mouth. 



In these species of Comasteridaj we find a perfect bilateral symmetrj-; an 

 anterior mouth midway between two exactly similar rays, a central anal proboscis, 

 and a dwarfed posterior ray with two exactly similar, sometimes more or less 

 dwarfed, rays, one on either side of it (figs. 27, 28, p. 69). 



There can be little doubt that this secondary bilateral symmetry in the Comas- 

 teridse is the direct result of the pressure resulting from the growth of the digestive 

 tube, a pressure which constantty tends to force the mouth to the right, the mouth 

 in its migration taking with it all the circumoral ring systems; for in comas terids 

 with a central mouth, and in the young of the other forms before the mouth has 

 begun to migrate, the five postradiai series are always similar and equal. 



The calyx plates of all the species of ComaSteridiB are so reduced that they 

 form merely a small central disk upon which, as well as upon the arm bases, the 

 visceral mass rests. This relationship between the calyx and the visceral mass is 

 common to the pentacrinites, the thiolliericr mites, and the comatulids, and in the 

 young comasterid is far advanced, in fact almost perfected, before the migration of 

 the mouth begins, §o that we are justified in assuming that it is phj'logenetically 

 much older than the beginnuigs of the additional coils of the digestive tube. Thus 

 it has not been possible for the coilhig of the digestive tube to exert anj' direct influ- 

 ence whatever upon the caljx plates or upon the arms, for whatever goes on within 

 the visceral mass is necessarily quite independent of the dorsal skeleton. 



The mouth is more or less fixed in position by the ambulacral structures which 

 lead to it; moreover, growth of the digestive tube whereby its length is increased 

 does not take place in the anterior, but in the posterior portion. Therefore the 

 lengthening of the digestive tube results in the formation of a sjiiral about the anal 

 proboscis as a center, tliis structure moving more and more centralward as the sj)iral 

 increases the number of its turns. 



The ring systems about the mouth, and their radial continuations to the arms, 

 are accommodated by a more or less vertical j)osition of the anterior part of the 

 digestive tube. The horizontal coils of the posterior portion of the digestive tube 

 about the anal proboscis as a center ])ress ujion the subambulacral systems miming 

 to the two posterior arms; these are therefore shoved to one side and come to lie m a 

 marginal position, forming a horseshoe about the anterior portion of the disk, where 

 they fuse more or less with the same structures running to the three anterior arms. 



