32 PHOEMOSOMA PLACENTA. 
coronal system. From seven to nine of the coronal plates appear at the 
same time, the division line of the sutiu-es 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 youngest stage (8 nun. 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 Cidarida?, 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 Avell 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 
Echinpids 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 gi'owth of the interambulacral areas combines to a certain 
extent the modes of formation of the plates of the abactinal system of 3'oung 
Starfishes and of young Ophiurans, in which the interambulacral plates are 
derived directly from tlie 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 tlie 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 pedicellarite are built up b}' 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 romul a central space, which eventually forms the 
perforation of the tubercle ; as this increases in size, the mammary boss is 
gi-adually formed from a similar concentration of the limestone meshes, 
and finally the edge of the boss becomes indistinctly crenulated. 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 as primary tubercles. 
