REPORT ON THE ECHINOIDEA. 75 



thuridse closely resemble what I have temporarily regarded as the young of Micropyga 

 tuherculata (PL XVIII/ figs. 10, 11; PI. VI." figs. 6, 7). We find there the actinal 

 membrane carrying ten buccal plates, with other plates in the extension of both the 

 ambulacral and iuterambulacral areas (PI. VI.^ fig. 7), but only the buccal plates and the 

 two ambulacral plates adjoining the test are perforated for tentacles, while in the yoimg 

 of Phormosoma, with a corresponding stage of development of the actinal membrane, 

 the ambulacral actinal plates are all perforated. In the young of Micropyga (PI. VII." 

 fig. 6) the apical system at an early stage shows the structure characteristic of the genus, 

 while the anal system is left comparatively bare of plates. This is not the case with 

 the apical system of the young of Phormosoma or of Asthenosoma. The anal system 

 (PL XVIIL" figs. 2, 5, 6 ; PL XII." figs. 2, 5, 7) is at an early stage covered by a large 

 number of distinct plates, while the genital ring of corresponding stages is, on the 

 contrary, often quite indistinct from the anal plates (see PL XVIII. " fig. 6); while in 

 other cases (PL XII.* figs. 5, 7 ; PL XVIII." fig. 3) the genital and ocular plates are 

 more distinct perhaps from the anal system than they are in older stages, especially in 

 those species where the genital plates show a tendency to split up into distinct plates, 

 the upper plates apparently belonging to the anal system, or where the genital plates 

 encroach far into the median interambulacral space^ at the abactinal extremity of the 

 test, separating completely the adjoining interambulacral plates. So that, judging from 

 the few young Echinothuridse wliich have been examined, the new plates of the actinal 

 membrane are primarily formed by becoming separated from the ambulacral plates of 

 the test, while additional interambulacral plates seem to be formed at the abactinal 

 pole from the pushing down of large marginal plates of the anal system on each side 

 of the genital plate. 



This whole development of the abactinal and actinal systems of the Echinothuridse 

 and Diadematidse, plainly shows a far closer structural relationship between them and 

 the coronal plates than is apparent when studying the regular Desmosticha, in which 

 the contrast between the compact abactinal system, and the nearly bare actinal 

 membrane with the coronal plates is most apparent. In fact, the structure of the aj)ical 

 and actinal systems of the Spatangoids and Clypeastroids shows the close connection 

 existing between them and the coronal plates, and also plainly shows that both these 

 systems are but modifications of the actinal and abactinal extremities of the coronal 

 plates, as is more plainly seen to be the case in the Starfishes and Ophiurans and in 

 the Holothurians, where the coronal plates are reduced to their simplest expression. 



The coronal plates of the youngest specimens of Pharmosorna (PL XVIII.' figs. 



' See the great extension of one of the genital plates in Heterodiadema libycum of Cotteau. This was supposed to 

 have been the anal opening. In Cotteau's figure (pi. xv., 5 fasc, Echin. Algerie) it is clear that it enters, as in the 

 Echinothuridse, deeply into the interambulacral spaces, but not all, only into one on account of the inequality in size 

 of the genital plates. 



