MUSEUM OF COMPARATIVE ZOOLOGY. 127 



According to Balfour,* the central calcareous plate appears after the 

 five radials and the five iuterradials. From this reference and the others 

 already mentioned, it would appear that in the starfishes, as far as ob- 

 served, the dorsocentral forms after radials and interradials. On account 

 of a discrepancy in the observations of Thomson and Gotte it is difficult 

 to compare the centrodorsal of Crinoids with the dorsocentral of the 

 Ophiuran, It would seem from Thomson's account as if the centrodorsal 

 was a single plate originating after the basals, but Gotte holds that the 

 centrodorsal is formed of a number of at first independent rods which 

 arise simultaneously with the basals. 



The shape of each of the five radialia may be seen by a consultation of 

 Figs. 13, 14. In the early condition of the plates the calcareous network 

 consists of a somewhat coarse, open reticulation, over which grows a finer 

 network connecting portions of the larger reticulation, and finely knitting 

 the whole together. This second kind of network does not appear in 

 figures of the primary radials and dorsocentral which have been given 

 by others. 



The primary radials, as shown by Ludwig, are not pushed out to the 

 end of the rays, as is believed by many naturalists. They always remain 

 on the abactinal hemisome of the body. It is therefore necessary to 

 modify what Balfour says on page 564,* of the formation of the plates of 

 Ophiurans, that " the original five radial plates remain as terminal seg- 

 ments of the adult rays." The original five radial plates are believed to 

 be the primary radials of the abactinal hemisome in Amphiura, and are 

 recognized in late stages on the body of the Ophiuran. 



Oral Plates. — There are certain plates (Fig. 18 6) situated in the inter- 

 radii, peripherally to the dorsal plates, or radialia at the very rim of 

 the disk, which, although belonging to the actinal hemisome, must be 

 mentioned here. They in point of fact originate on the abactinal hemi- 

 some, and by subsequent growth are carried to the actinal hemisome, 

 forming the oral shields. Their position of origin only will be spoken 

 of here, as a description of them will be given in an account of the plates 

 of the month {q. v.). 



The oral plates originate on the abactinal hemisome in the interradii 

 on the outer margin of the disk. The first, or one of the first, of these 

 to appear is perforated (Fig. 17 o), and according to Ludwig,t is a Tuadre- 

 porite or oral plate bearing the madreporic opening (see oral plates). 



Intermediate Plates. — Certain plates which are next to form on the 



* Comparative Embryology (Second Edition), p. 559. 

 t Op. cil., pp. 196, 197. 



