122 BULLETIN OF THE 



that the terminals appear at the same time as the radials, but could not 

 determine whetlier their origin is earlier or later in time. He considers 

 it probable that the terminals appear earlier. It is suggested that they 

 appear in Amphiura * later, as their comparative sizes (shown in my 

 figures) indicate. It is believed that Ludwig is right in his statement 

 that the centrale (dorsocentral) appears later than the five primary radials 

 in Amphiura. 



It is interesting to see how far this order of late development of the 

 dorsocentral is repeated in plates of the Asteroidea and Echinoidea sup- 

 posed to correspond with the dorsocentral, radials, and terminals. Our 

 knowledge of the sequence of the development of these plates is hardly 

 accurate enough to make definite statements, but there seems to be some 

 resemblance in these three groups in this particular. 



According to A. Agassiz,t the abactinal system of the young Echinoid 

 " consists of a single large plate, . . . and the new plates are added in a 

 spiral manner round the anal plate." This large abactinal plate is figured 

 in Fig. 28 1 and in several figures by Loven. It would appear from 

 this that they regard | the suranal plate as formed in sea-urchins of 

 this age before the oculars and genitals. A. Agassiz finds in Salenia an 

 adult genus with this plate, as in the young of some other genera ; and ac- 

 cording to Loven, § in this genus (Salenia) this plate is retained through 

 life, and instead of being a temporary is a permanent structure. A. Agas- 

 siz II has examined specimens of young Saleniee to obtain information in 

 regard to the suranal plate and its homology with the " single large anal 

 plate of the early stages of young Echini belonging to other families;" but 

 he finds that "the arrangement of the plates of the abactinal system does 

 not diflTer from that of the older specimens, the suranal being only pro- 



* Sladen (op. cit., p. 27) judrjea from the figures of larval Ophiurans which pass 

 through a pluteus stage, given by Agassiz, Metschnikoff, and Miiller, that it is more 

 probable that traces of the terminal plates appear before the first radials. In the 

 case of the Amphiura studied and figured by Metschnikoff, and possibly from the 

 figures of Miiller, he finds the reverse seems to occur. My observations on A. squa- 

 viata show that the radials are well formed before the terminals attain any great size, 

 or that the radials are of considerable size before the terminals have grown from a 

 simple spicular form. 



t Embryology of Echinoderms, Mem. Amer. Acad., IX. 1864, p. 12, Fig. 28. 



t Revision of the Echini, Mem. Mus. Covip. Zool., p. 280, PI. IX. Figs. 3, 6, 7, 8 ; 

 PI. X. Fig. 2. 



§ Lltudes sur les !6chinoidees, Kovgl. Svenska Vetenskaps Handlingar, Bandet 11, 

 No. 7. 



II Report on the Echini, Menu Mus. Comp. Zool., Vol. X. No. 1. 



