10 p. HERBERT CARPENTER. 



which also show the under basals very clearly. Speaking of 

 them as " costals,"^ he says, "II est A'rai que les conduits 

 efferents des organes de la generation, chez les Echinoidees 

 en possession, toutefois incertaine, des costales, eu sout 

 relegues definitivement chez les Asteriadees et rejetes dans le 

 perisome interradial." Ludwig continually speaks of the 

 " genital plates of Urchins and Asterids," though he evidently 

 intends to assert nothing but their homology in the two groups. 

 But in one passage," at any rate, he distinctly implies that 

 these plates are sometimes related to the genital openings in 

 Starfishes just as in the regular Urchins. The genital plates 

 or scales of Ophiurids are of another nature altogether, and 

 may correspond, as stated by Agassiz, to the interbrachial par- 

 titions of Starfishes;^ but they must not be confounded with 

 the so-called *' genitals " in this group and in the Urchins. As 

 regards the Asterids, the name appears to be altogether 

 undeserved ; while Loveu's observations, supported by those of 

 Sladen^ seem to indicate clearly that the plates in question 

 remain in the neighbourhood of the dorsocentral, like their 

 homologues in the Ophiurids and Urchins, though occasionally 

 further removed from it. The following passage from Agassiz 

 is interesting in this connection. " But we have, in a great 

 many genera of Starfishes, the central part of the disk, showing, 

 in the young stages only, as regular an arrangement of the 

 plates of the abactinal system as in any Ophiuran, though it is 

 lost in the adult.'' Sladen's observations show, however, that 

 this regular arrangement may persist for a considerable time; 

 aud he has wisely discarded the use of the word " genitals " 

 for the primary interradial plates of the abactinal system of a 

 Starfish. 



Let us now pass on to a consideration of those types in 

 which the uuder-basals, already noticed in the young Am- 

 phiura (PI. I, fig. 12; 2), are traceable in the apical system 



^ For some remarks ou tlie use of this name, see this Jouraal, vol. xviii, 

 1878, pp. 363—365. 



" ' Morphol. Stud.,' ii, p. 79. 



3 ' North American Starfislies,' pp. 93, 103. 



