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BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



from their common ancestor, but also suggest the original method of formation of 

 the division series as developed in the crinoids. 



The existence of the radianal and of anal x in the fossil crinoids and in the 

 pentacrinoids of the recent forms indicates the persistence of transitional character 

 between the crinoids and the urchins. 



Mr. Frank Springer has noticed that in the Crinoidea Flexibilia there is a 

 curious influence which has modified the bilateral symmetry of almost every genus, 

 always in the same way; the small infrabasal is almost invariably located under 

 the right posterior radial; the radianal originates under the right posterior radial 

 and migrates from this position upward until it disappears, but always keeps to 



the right of the median line of the posterior inter- 

 ambulacral area; the vertical series of plates arising 

 from anal x is affected by the same tendency which 

 persists long after the radianal has disappeared, and 

 leans to the right so that the vacant space is alwaj's 

 widest at the left. 



The modification and differentiation of the anal 

 area in the older fossil crinoids by the occurrence 

 of a radianal and of the so-called anal x, while in the 

 later and recent types the anal area is similar to the 

 other interradial areas, would seem to indicate that 

 a perfected radial symmetry was attained through a 

 condition in which the posterior interradial area 

 was distinguished by the existence of two plates not 

 occurring elsewhere, and therefore that primarily the 

 crinoids were bilaterally symmetrical animals which 

 attained radial symmetry through a shortening of 

 the body and a correlated centralization of the 

 various organs. Additional facts apparently sup- 

 porting this view are the stability and absence of 

 variation of the anterior arm, which is not infre- 

 quently absent (though no case has been reported in 

 which any of the other arms are absent), and the 



Fia. 99. LATERAL VIEW OF THE PROXI- 

 MAL PORTION OF A SPECIMEN OF CHLO- 

 EOMETRA EUGOSA FROM THE PHILIP- 

 PINE ISLANDS, SHOWING THE RELATIVE 



PROPORTIONS OF THE CIRRI, CENTEO- 

 DORSAL AND ARM BASES, AND THE 



ARRANGEMENT OF THE CIRRI ON THE bilateral behavior of variation affecting the other four 



arms. The evidence on these points seemed so conclu- 

 sive that I once suggested the possibility of the derivation of the echinoderms through 

 a bilateral ancestor with two pairs of lateral body processes, the (not infrequently 

 absent) anterior arm being explained as one-half of an additional pair interpolated 

 between the two processes of the anterior bilateral pair; and I suggested as repre- 

 senting a step toward such a condition such variants among the insects as possessed 

 an additional wing inserted anterior to one of the wings of the anterior pair. 



This theory appeared to have abundant palseontological support, and was 

 moreover emphasized by the fact that in six-rayed individuals the added ray is 

 almost invariably inserted behind the left posterior, thus again pointing to the 

 anal area as representing a true vegetative posterior region. 



