14 BULLETIN OF THE 



The different divisions grade into each other, and in early conditions 

 are not distinguishable. 



Dorsocentral. — The dorsocentral plate [dc) is one of the earliest to 

 form, and is one of the least modified in its growth, of all the abactinal 

 plates of the body. It is believed to be homologous with the dorso- 

 central of Amphiura. Especial attention was paid to the time when 

 the dorsocentral forms in Asterias, and the development or stage of 

 growth of the terminals and basals when it first appears. This is 

 believed to be an important fact in comparisons both with Ophiurans 

 and with Echinoids.* The younger the larva is, the greater is the 

 distance of this plate from the plutean anus, or blastopore.! 



In the youngest starfish (PL I. fig. 2) in which the dorsocentral was 

 observed, there were five terminals, t^-t'', and five genitals, ff^-g^. 

 On the periphery of each of the terminals there were two trifid spi- 

 cules, sp, which later developed into the terminal spines. The stel- 

 late form, or the position of the inten-adii, was mapped out by slight 

 indentations in the rim of the disk between the terminals. The termi- 

 nals have the form of simph* bifurcated and Y-shaped calcareous rods. 

 The position of origin of the dorsocentral is in a small space of the 

 dorsal or abactinal region enclosed by the second, third, fourth, and 

 fifth genitals. It is hemmed in by these plates except at one place, 

 the brachiolarian notch, which is an unclosed interval separating in 

 the present stage the first genital from the fifth terminal. 



The dorsocentral originates as a simple calcareous rod, or nodule, 



* I have already elsewhere (Bull. '^Ins. Comp. Zoul., XIIL, No. 4, pp. 122, 123) 

 devoted some space to a discussion of the time of appearance of this plate. 



t The dorsocentral of Asterias is never as near the blastopore as in Asterina. 

 This fact is mentioned as its neigliborhood in the latter has been used by Carpenter 

 in comparisons of the centrodorsal and dorsocentral. Facts in Asterias do not sup- 

 port the supposition, that, when the dorsocentral first appears in this genus, it is closer 

 to the blastopore than in older stages, as Carpenter says is the case in Asterina. 

 Tliis does not deny that it may not be nearer in Asterina, but it is not in Asterias. 

 In Asterias no anus is formed, but the blastopore, which never opens among the 

 limestone plates, is simply closed after absorption of the brachiohiria. I am in- 

 clined to the opinion that tlie blastopore does not become the permanent anus, 

 but have made no observation on this point except that in my species of Asterias 

 the blastopore (brachiolarian anus) is simply closed, and never migrates around 

 the rim of the young starfish to the abactinal side. A. Agassiz, however, con- 

 siders that there is a close approximation of the calcareous plates and the anus, 

 and says {op. at., p. 46) that the " anus undoubtedly discharges at tins time through 

 one of the many limestone cells." But later he says, " I am not able to state this 

 positively, never having seen from any point discharges of fecal matter." 



