NOTES ON ECHINODERM MORPHOLOGY. 9 



which the lower part of the cup is exclusively formed, the 

 radials not having yet appeared in the equatorial zone between 

 them and the orals. In the Asterids, too, the radials (PI. I, 

 iig. 15 ; 4) appear to develope late, long after the basals and 

 the terminals. There can, I think, be little doubt but that 

 these interradial plates, which are so often present in the 

 " rosette " of Ophiurids, are identical with the five " inter- 

 mediate " plates occupying a similar position in the young 

 Amphiura squaraata (PI. I, fig. 12; 3) as figured by 

 Ludwig. I have already pointed out ' that I regard the latter 

 as homologous with the basals of Crinoids which are marked 3 

 in all the diagrams given by Sladen and myself; and I have 

 therefore designated the proximal interradial plates in the 

 Ophiurid disc in the same manner, believing them to belong to 

 the primitive abactinal system of the Echinoderm. According 

 to Agassiz^ the genital plates of the adult Ophiurid are formed 

 _jfrom the angles of the primitive interradial plates. But the 

 plates which he included in the latter category lay outside 

 the circle of radial primaries, and not within the circle, between 

 these primaries and the dorsocentral.^ They are not com- 

 parable, therefore, to the proximal interradials in the abactinal 

 system of other Echinoderms. These are called basals in 

 Crinoids, and " genitals " in both Urchins and Starfishes. 

 The latter name is an unfortunate one, as the genital ducts 

 may have no relation to these plates even in an Urchin ; and 

 it is more than doubtful if any such relation exists in any 

 Starfish. Agassiz* expresses himself very cautiously as to the 

 identity of the primary interradials of the larva with the 

 *^ ovarian plates '^ of a one-year-old Starfish. Ludwig tells us 

 nothing on the subject, but Loven^ gives some excellent figures 

 representing the position of these abactinal interradials in 

 well-developed individuals of four species of Asterids, three of 



1 This Journal, vol. xxii, 1882, pp. 378—380. 



2 * North American Starfishes,' p. 93. 



3 See this Journal, vol. xxii, 1882, pp. 380, 381. Note. 

 •• 'North American Starfishes,' pp. 51, 52. 



* ' litudes sur les I^cliinoidees,' pp. 88, 89. 



