34 PHORMOSOMA PLACENTA. 



no one of which becomes more prominent than the others. This is the 

 case only in the Salenia? and in the young stages of Echinida?, 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 Asthenosoma varium, figured by 

 Ludwig,* is interesting, as it is the only species of Echinothuria in which the 

 genital ring is closed, connecting the structure of the apical s}-stem with 

 that of the Diadematidae proper. It is true that the specimens of which 

 Ludwig figures the abactinal system were not fully grown, and were much 

 smaller than the specimens of the allied species .4. Gruhi, figured in the 

 Echini of the Challenger Report, Plates XV.-XVIL, 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 Echinothuria^ 

 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 ambulacral 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 mm. in 

 diameter that we can find the characteristic generic arrangement of the 

 ambulacral 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., PI. II. 



