34 UINTACRINUS: ITS STRUCTURE AND RELATIONS. 



tlie axial nerve cords passing down from the radials would meet at about 

 the centres of the basals, and again fork toward the infrabasals ; while 

 in Form M, whether young or old, the lobes of the chambered organ 

 would be interradial, and the branches of the axial nerve cords from ad- 

 jacent radials would meet about the centres of the basals and not fork 

 again, but pass directly into the chambered organ. A transformation of 

 one form into the other, as a result of individual growth, would therefore 

 involve, in addition to the change in orientation of the centrale, a revolution 

 of the chambered organ, and an extension or shortening (as the case might 

 be) of the downward prolongations of the axial nerve cords. It may be 

 suggested that owing to the absence of a stem such changes might be 

 accomplished with less difficulty than would exist in stalked Crlnoids. But 

 it seems probable, from analogy with the life history o? Antedon, and what 

 we know of Adinnmetra merklionalis, that these Crinoids had stems in the 

 larval stage. And it is difficult to imagine how such changes could jiossibly 

 have taken place, in either stalked or unstalked Crinoids, after the larval or 

 pentacrinoid stage was passed, and the position of the hard parts of the 

 skeleton became fixed. 



Furthermore, none of the above methods would account for the focts as 

 we find them. What shall be said of cases such as are presented by Figs. 

 1 and 2 of PI. III. ; 6 and 7 of PI. V. ; and 1, 2, and 6 of PI. VI., where 

 the two forms of base are found fully developed in specimens of the largest 

 size, or by Figs. 3, 4, and 5 of PI. III., where they are equally distinct in 

 very young individuals? These are not isolated cases, as may be seen 

 fi'om the Table F infra, which shows that the two forms occur indiscrim- 

 inately among old and young, witli a considerable predominance, however, 

 of the dicyclic base among the younger specimens, and a slight excess of 

 the monocyclic base among the older ones. Among specimens measuring 

 under 25 mm. across the calyx, 75 per cent are dicyclic; while among 

 those of maximum size — 50 mm. and over — the proportion of dicyclic 

 is about 40 per cent. If there were a tendency to develop by individual 

 growth into either form, then we ought to find practically all the adult 

 specimens belonging to that form ; and the same thing would be true if 

 the transition took place during the larval stage. The great number of 

 well-developed individuals of each type — so great that, as before remarked, 

 we do not know positively which was the normal form, or if there was one 

 — is in my opinion a fact totally inconsistent with any of the above sup- 



