160 



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 nrcluns. 



Mr. Frank Sjiringer has noticed that in the Crmoidea Flcxibiha there is a 

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

 alwaj's in the same wa}'; 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 hne of the posterior intcr- 

 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 

 loans to the right so that the vacant space is always 

 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 mterradial 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 plati'S not 

 occurring elsewhere, and therefore that prunarily the 

 crmoids were bilaterally sjanmetrical animals which 

 attained radial symmetry through a shortening of 

 the body and a correlated centralization of the 

 various organs. Additional facts apparently sup- 

 I)orting tliis \-iew are the stabiUty and (vbsenco of 

 variation of the anterior arm, which is not infre- 

 quently absent (though no case has been reported in 

 which an}- of the other arms are absent), and the 

 bilateral behavior of variation affecting the other four 

 arms. The e\adonce on these points seemed so conclu- 

 sive that I t)nco suggested the possibiUty 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- 

 sentmg 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 ])air. 



This theory appeared to have abundant palieontological su])port, and was 

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

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

 anal area as rejjresenting a true vegetative posterior region. 



FlO. 99.~I,ATERAI, VIEW OF THE PROXI- 

 MAL PORTION OF A SPECIMEN OF CHLO- 

 ROMETRA RUGOSA FROM THE rutLIP- 



PixE Islands, anowiNO the relative 



PROPORTIONS OF THE CIRRI, CENTRO- 

 DORSAL AND ARM BASES, AND TBE 

 ARRANGEMENT OP THE CIRRI ON THE 

 CENTRODORSAL. 



