HOMOLOGIES OF ECHIISODERMS. 



Comparatively little use has been made thus far, in the study of 

 Starfishes, of the structure of their hard parts. With the exception of 

 the short paper by Gaudry, the occasional references to them in sundry 

 memoirs of Miiller, Duvernoy, Agassiz, Perrier and others, only show 

 that we know but little of the solid frame concealed under the mass 

 of appendages covering a starfish. The study of these solid parts is 

 instructive, as it throws new light on their homologies with other Echi- 

 noderms, and enables ns to form a better idea of the I'elationship be- 

 tween Ophiurans, Echini, and Starfishes. The homology between these 

 orders, as usually understood, can be stated in a general way as follows ; 

 the great development in Starfishes of what has been called the tergal 

 system, covering the centre and arms, forming a system where no special 

 regular arrangement could be traced, joined to the presence of distinct 

 ambulacral and interambulacral plates, limited to the furrows occupying 

 the lower face of the arm ; also the absence of specialized genital plates, 

 or anal plates, and the presence (in some genera) of a special ocular 

 plate. The essential character in a general way of the Ophiurans as 

 distinguished from Starfishes is the presence of genital plates, and the 

 limitation of the tergal plates to a comparatively simple casing, con- 

 sisting of few plates enclosing an ambulacral system, no interambulacral 

 system having been traced ; while in Echini the tergal system is re- 

 duced to a minimum, ocular and genital plates being present, and the 

 ambulacral and especially the interambulacral plates greatly developed. 



From the more careful examination of Echini a hypothetical Echino- 

 derm was formerly established, which has served as the type to which 

 the other Echinoderms were to be reduced, and with which they should 

 homologize. In the absence of embryonic data, however, the fundamental 

 facts derived from the study of young Echinoderms already point to 



