o 



42 W. PERCY SLADEN. 



Ophiurids being developed before the basals, and in Asterids 

 the terminals are formed before the radials. 



In both Asterid and Ophiurid the representatives of the 

 under-basals do not appear until the other plates are well 

 developed. 



The occurrence of under-basals in the Asterid as well as in 

 the Ophiurid larvae nullifies the argument advanced by Studer ^ 

 that the arrangement of primitive plates in Asterids and 

 Echinoids corresponds with the monocyclic Crinoids, whilst 

 that of Ophiurids corresponds with the dicyclic Crinoids. 



It is obviously premature to construct reliable hypotheses 

 solely upon the basis of the comparisons above instituted. 

 Our information is still too limited ; and an extended series of 

 observations upon the developmental stages of a larger number 

 of forms in each group of the Echinodermata is necessary 

 before the validity of any deductions can be assured. The 

 facts, however, above enumerated are very significant and 

 suggest at least the conclusion that the Ophiurid derives its 

 origin from a more highly developed Crinoid ancestor than the 

 Asterid, whose characters it has also retained more preponder- 

 ately ; that the Asterid was in all probability derived from a 

 Crinoid of more primitive type ; and that the two forms 

 advanced along collateral lines of descent. Upon the question 

 of the priority of origin, it is still premature to invoke embryo- 

 logical evidence. 



1 "Uebers. ueb. die Ophiuriden welche wahrend der Reise S. M. S. Gazelle 

 ge.sammelt warden," p. 10, ' Abliaudl. d. Kouigl. Preuss. Akad. d. Wissensch. 

 zu Berlin, vom Jahre 18S2.' 



