158 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Now the effect of the movements of the posterior end of the digestive tube 

 upon the progressive reduction in the size of the calyx, and upon the reduction of 

 the number of the calyx plates, is continually to hinder its progress in the posterior 

 interradial or anal area, so that this area constantly remains somewhat larger than 

 the others and is the last one from which the primitive calyx plates, having become 

 functionless and obsolete, are dropped. The lateral and ventral movements in the 

 posterior end of the digestive tube cause a continual lifting stress, which is exerted 

 in a diagonal direction toward the upper right-hand corner of the posterior inter- 

 radial or anal area, or more correctly result in propping up this corner of the pos- 

 terior interradial area, as well as the right posterior postradial series, so as greatly 

 to hinder the consummation of the reductive processes. 



As a consequence of this force, always present and constantly exerted, the 

 interradial and other plates in the posterior interradial area are able to maintain 

 their individuality and their existence long after they have entirely disappeared 

 from all the other areas, while as a result of the constant propping up of the right 

 posterior ray the subradial plate is able to maintain itself under that ray long 

 after it has disappeared from beneath all of the others ; at the same time the tend- 

 ency to reduction, which is just as strong in the posterior as in the other inter- 

 radial areas, will be confined to the left-hand side of that area, so that all of the 

 plates and structures lying in it will be distorted and turned toward the right. 



The presence of the persistent subradial plate under the right posterior radial 

 is a characteristic feature of many genera in the Flexibilia, and, so far as is known, 

 this plate is always present in the young of the recent forms (fig. 563, pi. 6). But 

 its true significance and its homologies have heretofore never been understood; in 

 the fossil types it has been considered a distinct entity and dignified by the name 

 of radianal, while in the recent types, as for instance in Antedon, it has always been 

 known as the anal, though it has nothing whatever to do with the so-called anal 

 of the fossil species. 



The observed tendencies in the species of the fossil Crinoidea Flexibilia, and 

 the effects which we would naturally infer would follow in crinoids undergoing 

 reduction in the size of the visceral mass and of the calyx which possess a digestive 

 tube of the type occurring in the recent species (excepting certain comasterids) for 

 purely mechanical reasons, are thus seen to be in perfect agreement. 



As the entire test of the urchin, except for its small apical portion, is comparable 

 to that part of the crinoid between the apical system and the arm bases, it naturally 

 follows that any increase in the plates of the latter in this intermediate area is a 

 step in the direction of the urchins. 



The radial is the equivalent of two of the ambulacrals of the urchins; the 

 radianal (or any one of the subradials) is the counterpart of another (single) ambu- 

 lacral formed between the radial, which represents the two radial ambulacrals border- 

 ing the peristome, and the infrabasal, which represents the ocular. 



Thus the subradials of the crinoids are formed exactly in the same place and 

 in the same manner as the series of ambulacrals in the echinoids, and they not only 

 give us a valuable clew to the paths of divergence of the crinoids and of the echinoids 



