368 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



to increase in number as they do indefinitol}- in the urchins, shifted about and 

 fused in such a way as to meet all the necessary mechanical requirements without 

 increase in number. 



Increase in the number of the anns in the crinoids, at least in the pentacrinites 

 and comalulids, is accomphshcd by a curiously imlirect and wasteful nietliod. The 

 original arms break off, typically between the first two brachials, and additional 

 division scries arc formed, the last giving rise to new arms which are the exact equiva^ 

 lents to the arms cast ofT. This curious interpolation of division series between the 

 base of the original arms and the base of the adult arms is the only remaining vestige 

 of the method by which the division series were originally formed. 



In the crinoids the development of ambulacrals comparable to those in the 

 echinoids ceased abruptly, while the development of true ambulacrals (brachials) 

 beyond them was carried to an extreme. In the urchins the "ambulacrals" have 

 developed to such an extent that they encompass the entire lateral surface of the 

 animal except for a small area about the mouth, while only the first beginnings of 

 true ambulacrals are found, in the shape of 10 more or less developed processes 

 within the body cavity about the peristomic area. 



If we imagined an urchin in which the skeleton formation had been suddenly 

 arrested so that the peristome was expanded as far as the ambitus, and in which 

 the auricles had become turned outward and extraordinarily developed through 

 the consumption of the energy which normally would have been used in the de- 

 velopment of ambulacrals, we should have a creature which, in so far as the skeleton 

 is concerned, would be a crinoid. We should merely have to move the anus to the 

 perisomic ventral surface, develop the suranal plate into a column, change the 

 teeth from their highly specialized form into generalized oral plates lying in the 

 integument, segregate the ambulacrals and bring the enormously enlarged auricles 

 into lateral contact, carrying out the ambulacral structures upon their ventral 

 surface, to make our crinoid perfect. 



It is to the development of the column and its mechanical effects on the animal 

 that attention must chiefly be directed. The development of a column from the 

 suranal j)lato would first of all cause the coronal ring of plates to contract, so that the 

 animal would rest with the column supporting the plates of the coronal ring instead 

 of pushing upon the internal organs. In this contraction of the coronal ring five 

 of the plates would form one circlet, and five another circlet, the plates of the latter 

 alternating with those of the former. In the echinoids there is a gradual enlarge- 

 ment of the coronal ring; at the same time the plates composing it gradually enlarge 

 so that the ultimate arrangement becomes five large genitals immediately sur- 

 rounding the pcriproct with five small oculars between their distal cornel's. This is 

 the result not of any change in the relative position of the plates but of their pro- 

 portionate growth inward by accretion along their free edges over the periproctal 

 area. The large genii al plates naturally grow faster than the small ocular plates 

 and eventually come into contact behind them, excluding them entirely from the 

 periproct (figs. 71, 73, p. 127), but without in the slightest degree altering the inter- 

 relationships of the original calcareous ossicles. If a contraction in the coronal 

 ring of five large and five small plates, such as would become necessary upon the 



