DIADEMATID.^. 107 



and the poriferous zone. In life, they are probably more or less pendent; 

 but in preserved, and especially dried specimens, they are often quite erect. 

 The calcareous particles of the Diadematidte are quite characteristic in 

 that they are fundamentally triradiate, and in their simplest condition they 

 exhibit this form very prettily. Such spicules have three branches of equal 

 length, equally distant from each other, perfectly straight, smooth, and 

 pointed; they are to be found chiefly near the tips of the pedicels, and most 

 commonly on the abactinal surface. Usually, however, the branches are 

 not equal, nor are they straight, and they often have subordinate branches 

 growing out from them (PI. 50, fig. 31). As these branches become more 

 numerous (PI. 51, fig. 11), they tend to coalesce and make small, irregular 

 plates with few perforations (PI. 50, fig. 19) ; these occur in the lower 

 part of the pedicels, or in those of the actinal surf\ice, or sometimes in the 

 gills. These plates may continue their growth in either of two different 

 ways : they may elongate, but remain narrow, increase in thickness, and 

 decrease the diameter of the perforations, and thus become such supporting 

 plates as occur in the pedicels, especially of the actinal surface, of many 

 individuals ; or they may increase in diameter irregularly in all directions, 

 remaining thin and with large perforations, and thus become the fenestrated 

 plates which occur commonly in the gills, and sometimes in the pedicels, of 

 many species. These plates sometimes grow to a large size, in the basal 

 part of the gills becoming large enough to be seen with the unaided eye. 

 In Micropyga, one branch of the fundamental triradiate spicule becomes 

 greatly elongated, and either forms the handle of the characteristic anchor- 

 shaped particles found in the pedicels of that genus, or becomes a rod, ex- 

 panded and fenestrated at each end and in the middle; these rods often show 

 their triradiate origin plainly, and not infrequently develop into irregular 

 perforated plates. As age and condition appear to affect profoundly the size 

 and number of the calcareous particles, we fail to find in them any satisfac- 

 torily constant generic or specific characters, except, of course, in Micropyga. 

 The arrangement of the internal organs in Chaitodiadema (PI. 44, figs. 5, 6) 

 is illustrative of the whole family ; for a careful examination of specimens 

 of Diadema, Echinothrix, Astropyga, and Micropyga reveals no important 

 character in which these genera differ from each other. The reproductive 

 organs are very dense masses of short tubules, which occupy a greater or 

 less proportion of each interambulacrum according to their maturity. The 

 alimentary canal is remarkable for its very great development, its total 



