MUSEUM OF COMPARATIVE ZOOLOGY. 121 



siz,* MetschnikofF,t and Ludwig.J The dorsocentral early takes its per- 

 manent place, and remains near the centre of the abactinal region of the 

 disk ; the radialia are subsequently to formation pushed nearer the peri- 

 phery by the interposition of new plates between them and the dorsocen- 

 tral. They are never, however, in Amphiura pressed to the very periphery 

 of the body, and consequently never extend out on the arms. In early 

 stages the dorsocentral § is not formed, the five radialia originating first. 



The dorsocentral in the young Amphiura (Fig. 1 3) occupies the centre 

 of the abactinal surface of the body, and is irregularly pentagonal. Its 

 origin was not traced, but there is no reason to question the observations 

 of Ludwigt and Schultze|| that the radials appear before the dorsocentral. 

 In the youngest pentagonal embryo which was studied (Fig. 12) in which 

 the umbilicus {u) was still unbroken, the five radialia (rp) were well 

 formed and of comparatively large size, while the dorsocentral (dc) was 

 small, indicating apparently a later formation. Ludwig t says, " Ganz 

 unterdriick aber wird die Entwickelung eines Centrale niemals." The 

 radialia or primary radials are among the first plates to form, and origi- 

 nate while yet the Amphiura is in a bilaterally symmetrical condition. 

 It is extremely difficult to distinguish them in earliest conditions from 

 the network at the anal end of the provisional pluteus rods (Fig. 8), but 

 they are among the first plates of the Amphiura to form. The question 

 of whether they antedate in formation any of the plates of the actinal 

 region of the disk will probably be correctly answered in the affirmative. 

 They probably appear before the first pair of adambulacral (q. v.) plates 

 and undoubtedly before the terminalia (q. v.). 



A. Agassiz states :1[ " We have the most positive proof of the origin of 

 the dorsocentral plate of starfishes and Ophiurans as a single Y-shaped rod 

 appearing simultaneously with the five basals." Ludwig, on the other 

 hand, states : " Diese sechste Platte, das Centrale tritt bei Amphiura squa- 

 mata in der Eegel spater auf als die fiinf Eadialia." Ludwig holds ** 



* Embryology of Echinoderms, Mem. Amer. Acad. Arts and Sci., III. 1856. 



t Op. ciL, p. 18 ; PI. IV. Fig. 17. 



t Op. ciL,-p. 195. 



§ The reader is referred in this connection to Fig. 16, PI. VII., in my paperon the 

 development of Echinarachnius {Bull. Mus. Comp. Znol., XII. 4). In this figure, 

 which is of a young sea-urchin, the calcareous network of the dorsocentral plate {c pi) 

 has not begun to form, although that of the ring of plates surrounding it is well 

 marked. 



II Op. ciL, PL I. Fig. 4. 



IF Report on the Echini, Mem. Mus. Comp. Zool., Vol. X. No. 1. 

 ** Page 187. 



