34 PHORMOSOMA PLACENTA. 
no one of which becomes more prominent than tlie others. This is the 
case only in the Salenire and in the young stages of Echinidte, and does 
not seem to have reached as yet its greatest development in the Echini 
of the present period. Unfortunately there is no Pluteus of Echinus figured 
in a stage showing the first appearance of the calcareous rods. Such an 
observation might throw very important light on the homology of the 
Echinoid apical system. 
The apical system of two specimens of Asfhemsonia varium, figured by 
Ludwig,* is interesting, as it is tlie only species of Echinothuria in which the 
genital ring is closed, connecting the structure of the apical system with 
that of the Diadematidas proper. It is true that the specimens of Avhich 
Ludwig figures the abactinal system were not fully grown, and were much 
smaller than the specimens of the allied species A. Gruhci, figured iu the 
Echini of the Challenger Report, Plates XV.-XVII., in which the genital ring 
is less open than in any other species of Asthenosoma I have had occasion 
to examine. It will be seen, on examining the figures of young Echiuothuria3 
in the Report on the Echini of the Challenger, that, while the plates of the 
genital ring are placed nearer together in the young stages than in older 
specimens, yet they are more or less separated even in the earlier stages by 
the anal plates, which force their way between the ocular plates and the two 
sides of the genital plates in the two zones of the apical part of the inter- 
ambulacral area. 
In the early stages (8 mm. in diameter), the ambulacra! tentacles are 
arranged in a single vertical row, extending from the actinal opening to the 
ocular plate. It is only in specimens measuring from 28 to 40 nun. in 
diameter that we can find the characteristic generic arrangement of the 
ambulacra] tubes. In specimens measuring from 17 to 25 mm. in diameter, 
the actinal imbricating plates have increased to four, the gills are well de- 
veloped, and the final arrangement of the primary tubercles both of the 
actinal and abactinal surface is indicated in a general way on the plates 
adjoining the ambitus, in both the ambulacral and interambulacral areas. 
The marginal fasciole can already be made out in specimens measuring 
about 28 mm. in diameter ; in specimens of 50 mm. in diameter it is quite 
prominent. 
The lapping of the coronal plates takes place at a later stage of growth 
than that of the plates of the actinal membrane ; the latter are distinctly 
* Zeitsch. f. Wiss. Zool., XXXIV., PL II. 
