122 BULLETIN OF THE 
that the terminals appear at the same time as the radials, but could not 
determine whether 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+ and in several figures by Lovén. 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 Lovén, § in this genus (Salenia) this plate is retained through 
life, and instead of being a temporary is a permanent structure. A. Agas- 
siz || has examined specimens of young Saleniz 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 differ from that of the older specimens, the suranal being only pro- 
* Sladen (op. cit., p. 27) judges 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- 
mata 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. 
+ Embryology of Echinoderms, Mem. Amer. Acad., IX. 1864, p. 12, Fig. 28. 
+ Revision of the Echini, Mem. Mus. Comp. Zodl., p. 280, Pl. IX. Figs. 3, 6, 7, 8 ; 
Pl. X. Fig. 2. 
§ Etudes sur les Echinoidées, Kongl. Svenska Vetenskaps Handlingar, Bandet 11, 
Nos 7. 
| Report on the Echini, Mem. Mus. Comp. Zodl., Vol. X. No. 1. 
