MUSEUM OF COMPARATIVE ZOOLOGY. 15 



which immediately grows into a trifid spicule. As the starfish matures 

 the dorsocentral becomes branched (PI. I. fig. 3), forming a flat pen- 

 tagonal plate occupying the central region of the abactinal part of the 

 body of the starfish. 



As the starfish grows older, the dorsocentral is found to carry a large, 

 well marked, centrally placed spine (PI. II. fig. 4) ; later, two and three 

 other spines of large size form. These spines are relatively to the size 

 of the starfish much larger than those of the adult. 



In the oldest larval starfish (PI. IV. fig. 4) which was studied, the 

 dorsocentral still preserves its pentagonal form, and, on a line passing 

 through the dorsal region of the starfish arm, from its angles arise the 

 radials of the disk. The dorsocentral always preserves its central posi- 

 tion even into the adult starfish, and never undergoes any considerable 

 modification in outline or in size, although of conrse its size relative to 

 that of the starfish is smaller as the starfish matures. 



The time of development of the dorsocentral in Amphiura is after the 

 primary radials and basals.* As there are at first no plates which can 

 be compared with the radials in Asterias, we can simply say that the 

 dorsocentral is formed in the starfish after the basals ; but if we com- 

 pare the first median dorsal arm-plate of the starfish with the radial of 

 Amphiura, we must say that the dorsocentral originates before the first 

 radial in the starfish. This seems to me an additional argument, 

 although I confess not a strong one, against considering the first median 

 dorsal arm-plate as homologous with the radialia, or primary radial, of 

 Amphiura. Still it would seem that the relative time when plates 

 appear in the Echinoderms is unimportant, as far as a determination of 

 their morphology is concerned. My observations on the time the dorso- 

 central plate appears, as compared with the ten primary plates of Aste- 

 rias, support the statements of A. Agassiz on this point. 



Terminals. — The terminals, t l — t b , are the most conspicuous and 

 largest of all the primary plates in the embryonic life of the starfish. 

 While the starfish is yet in the brachiolarian stage (PI. I. fig. 1) the 

 terminals appear, and in the oldest form considered they are still promi- 

 nent. From first to last, then, these plates are important calcifications 

 in the growing Asterias and in the modifications of its form. There are 

 five terminals. These will be designated in the following way. Begin- 

 ning with the madreporic opening, t\ and ending near the same body, 

 t 5 , passing around the anal pole of the brachiolaria from dorsal to 

 ventral side, they extend through a complete semicircle. The termi- 

 * Bull. Mus. Comp. Zool., XIII., No. 4, p. 121. 



