ON THE YOUNG STAGES OF ECHINI. 733 



Nowhere among the young regular Echini have I found such great 

 changes in the shape and proportions of the test and spines as in Echi- 

 nometra. We frequently find specimens of the same size, where in one case 

 the outline is almost circular, the test flattened, covered with long slender 

 spines, while in the other the test is lobed, swollen, high, surmounted by 

 numerous short stout spines {PI. X". f. 2, s). These and all intermediate 

 stages, complicated by the greater or smaller number of primary tubercles, 

 are found retained in specimens of very different size; the arrangement of 

 the arcs of the poriferous zone undergo changes exactly similar to those 

 described in Strongylocentrotus. This has given rise in a great measure to 

 the confused synonymy attached to the most common species of the genus, 

 and renders their identification, if based upon meagre material, almost hopeless. 



In young Arbaciadae {PL V. f. 9) we have already in the youngest stages 

 four anal plates. I have traced {Fig. as) in the youngest Arbacia raised 

 from the pluteus the first appearance of the anal plates, which appear simul- 

 taneously as four lines radiating from the apex, and forming the separations 

 of the four anal plates (see Fig. as). The abactinal system of young speci- 

 mens is remarkably prominent, occupying more than one half the abactinal 

 part of the test. The whole test is deeply pitted {PI. V. f. 11), Trigono- 

 cidaris-like ; the rudimentary tubercles, covering the greater part of the 

 abactinal part of the test, are connected by ridges, which are gradually re- 

 sorbecl and reduced to the granulation found upon the coronal plates of the 

 genus {PI. V. f. 2, 14, 16, 18). The primary tubercles are at first limited 

 to the ambitus {Fig. 69), surmounted by short stout spines {PI. V. f. 9), 

 Colobocentrotus-like, gradually becoming more slender and proportionally 

 longer with increasing age {PI. V. f. 12, 14, 17), — the opposite of what takes 

 place in Strongylocentrotus, Cidaris, and most young Echini. The rudimen- 

 tary spines are not seated upon tubercles ; they are club-shaped (PI. V. f. 9), 

 identical in structure to those of Podocidaris {PI. IV. f. 1.:). The poriferous 

 zone has in the earliest stages the structure found in the adult, only it does 

 not widen at the actinostome {PL V. f. 15). The ratio of the actinostome to 

 the test does not vary greatly in different stages of growth ; the edge of the 

 actinal system forming the groove of the gills is but slightly turned back in 

 the young, the lips, taking the place of cuts, becoming more prominent 

 (Toxopneustes-like) with increasing age {PL P. f. a). Owing to the inde- 

 pendent growth of the plates of the poriferous zone from those of the 

 interambulacral system, we have either three or four pairs of pores for each 



