32 PHOEMOSOMA PLACENTA. 



coronal system. From seven to nine of the coronal plates appear at the 

 same time, the division line of the sutures being traced with difficulty in 

 young specimens measuring 8 mm. in diameter, but becoming well defined 

 in somewhat larger specimens of from 17 to 20 mm. in diameter. 



In the voungest stage ( S mm. in diameter) the actinal plates are separated 

 from the coronal plates, and are developed, as I have shown, in the same 

 manner as the imbricating plates of the Cidaridae, independently of the 

 coronal plates; new plates forming on the distal surface of the actinostome, 

 which are intercalated between the old plates and the coronal plates. On 

 the abactinal system, on the contrary, while the plates of the genital ring 

 are well defined and seem to be distinctly separated from the coronal plates, 

 yet new interambulacral plates are not added independently, as in the 

 ambulacral system, and as in the interambulacral system of other young 

 Echinoids where the genital ring remains permanently closed. The new 

 interambulacral plates are found to be pushing out from the plates of the 

 anal system on each side of the genital plates. As the ocular and genital 

 plates of the genital ring become separated, with increasing size, the addi- 

 tional anal plates formed in the intervening spaces are pushed out. and 

 become a part of the abactinal portion of the interambulacral area. 



This mode of growth of the interambulacral areas combines to a certain 

 extent the modes of formation of the plates of the abactinal system of young 

 Starfishes and of young Ophiurans, in which the interambulacral plates are 

 derived directly from the plates of the abactinal system. This shows a far 

 closer relationship between the young of some of the Sea-urchins of the 

 present day with Starfishes and Ophiurans on the one side, and Holothurians 

 on the other, than had been suspected formerly. The plates are formed by 

 a close-meshed reticulation of small, comparatively thick Y-shaped rods, 

 with bare interstices for the joints of the plates. The tubercles of the 

 miliaries and of the pedicellarise are built up by a close accumulation of 

 these meshes, forming what appears a fine granulation. The small tuber- 

 cles are formed by the clustering together of five or six larger cells, 

 arranged concentrically round a central space, which eventually forms the 

 perforation of the tubercle; as this increases in size, the mammary boss is 

 gradually formed from a similar concentration of the limestone meshes, 

 and finally the edge of the boss becomes indistinctly crcnulated. The 

 tubercles, when arrested in their development in any one of these success- 

 ive stages, form what are known as miliary, as secondary small primary, 

 and us primary tubercle-. 



