238 



that all the plates of similar Starfishes, which Gaudry does not hesi- 

 tate to consider as interambulacra, have nothing in common with either 

 the ambulacral or interambulacral region, but are a part of the abac- 

 tinal system ; showing thus plainly that we may have in the abactinal 

 system a differentiation of parts aping structural features of a system 

 in Sea-urchins, with which the system to which these plates belong 

 has nothing in common ; showins us, also, that those Starfishes which 

 thus ape this system of plates have a specialization of parts which 

 places them higher than the Starfishes in which these plates are 

 either wanting or only slightly developed. In the same way, from 

 a want of precision in his definition, Gaudry finds interambulacral 

 plates in Ophiurans, while the absence of an interambulacral system 

 along the ambulacral plates is the ordinal character separating them 

 from the Starfishes proper, and it would be more natural to associate 

 the Ophiurans with the Crinoids than with the true Starfishes. 



The great difference in the composition of the abactinal area in 

 different families of Starfishes requires some further discrimination. 

 The moment it is understood that the so-called marginal plates of 

 Astropecten, Astrogo?iiimi, Goniodiscus, &c., belong to the abactinal 

 area, it appears at once that there is great uniformity in the composi- 

 tion of the whole of that area in such Starfishes as Asteracanthion 

 and allied genera, while in the above-named genera the abactinal 

 area is subdivided into distinct regions, the plates between the mar- 

 ginal plates and those of the actinal area differing more or less from 

 the marginal plates themselves, as well as from those occupying the 

 upper side of the Starfish. I propose, therefore, to designate them 

 by special names, calling Paractinal those which occupy the space 

 between the mouth, the ambulacral furrows and the marginal 

 plates; Metactinal, those commonly called marginal, but for which 

 this name cannot be retained, since there exists in Solaster a row of 

 specialized plates homologous to the marginal plates, which are not 

 along the margin of the rays ; and Epiactinal, the plates of the 

 upper surface, among which we may further distinguish the middle 

 row opposite the ambulacra, Antiactinal, and secondary rows parallel 

 to that, as well as the specialized rows, Meiagenial^ which may extend 

 from the angle between the rays toward the centre of the abactinal 

 area ; then, again, we may notice specialized plates surrounding the 

 anal aperture, and others opposite the angle of the mouth, Antistomal 



Finally, there exist among Starfishes peculiar plates, affording 

 excellent generic characteristics, which have been entirely overlooked, 

 extending between the upper and lower floor, in the direction of the 

 interval between adjacent rays, which I propose to call Diactinal 

 plates ; sometimes they form a wide partition, dividing the interior of 

 a Starfish into distinct chambers, while in others they form more or 

 less extensive bridges between the two floors. 



