48 EMBRYOLOGY OF THE STARFISH. 



^2)5 but not so broad at the extremity, and somewhat more slender. 

 New spines are always added between those originally at the extremity 

 of the rays and the centre of the disk ; the latter always remain the 

 most advanced and most prominent of the spines, even when the young 

 Starfish has assumed many more of the features of the adult than it has 

 at present, and has reached a stage when it would not be mistaken for 

 anything but a Starfish, closely allied to our common species. 



Network of Limestone Cells. — As we have seen in the earliest stages of 

 the Starfish, there are, on the abactinal area, rods from which, by the 

 addition of Y-shaped processes, clusters of polygonal cells are gradually 

 formed (PI. VII. Fig. 7); one cluster in the middle of each ray (PI. VI. 

 Fiy. 10, 4)j one around the smaller rod placed in the angle of the rays 

 (4), and a still smaller one round the rod placed in the very centre of 

 the abactinal area {l\ The large clusters extend and unite along the 

 edge of the rays, forming a continuous network ; it is from the cells of 

 the edge that the limestone deposit is formed, which extends over the 

 abactinal surface. The clusters of cells placed in the angle of the rays do 

 not unite laterally, though they become indirectly connected in the more 

 advanced stages of our Starfish, joining with the plates of the rays by a 

 few cells (PI. VI. Fig. 10). The central plate remains unconnected with 

 the others in the most advanced of the young which I have raised from 

 the Brachiolaria. The whole of the network is quite movable, and the 

 plates, before they become united, are capable of independent motion by 

 the contraction of different portions of the abactinal area. 



[Loven has given excellent figures of the young of Asterias glacialis, 

 corresponding to some of the stages here figured. They differ, however, 

 in having the plates more distinctly separated even in the young stages 

 (PI. VI. Fig. 10). The reticulation is compact, so that it is oidy in cer- 

 tain stages of expansion that the original composition of the abactinal 

 surface can still be traced. 



It is from the careful comparison of these young stages (Pis. VI. VII. 

 VIII.) with the corresponding stages of the 3'oung Ophiurans, given by 

 Metschnikoff, PI. IV. of his memoirs, and in my memoir on the Embrj'- 

 ology of Echinoderms, Figs. 29, 32 (Mem. Am. Acad.), and of young 

 Echini with the young of Comatida figured by Allman, Carpenter, and 

 Thomson, that we can make out a satisfactory homology of the test of 

 Echinoderms as has been so successfully done b}' Loven in his superb 



