ON THE YOUNG STAGES OF ECHINI. 



Figs. G8, 69. Young Arbacia punctulata, diam. 1.5mm. including spines- 



735 



Fig. 69. 



In Echinus, Sphaerechinus, Toxopneustes (PL VII., VIII.), we find in the 

 younger stages the same unbroken vertical arrangement of the pores (PL 

 VII. f. 8, ll), taking next a vertically arched form (PI. VII. f. 2, 4, 15, :.')■ 

 still connected, and then assuming the arrangement of the adult, In these 

 genera the anal system is at first covered by one plate only (PL VII. f. 9), 

 and undergoes changes similar to those of Strongylocentrotus, by the addition 

 at first of four smaller plates (PL VII. f. 1, 10), and so on (PL VII. f. 

 5, 17, 18), the original subanal plate retaining long a greater prominence 

 (PL VII. f. .;, e). The miliaries, in these genera as well as in Strongylocen- 

 trotus, are formed by radiating ridges arising from the base of the primary 

 tubercles, forming a sort of star; then they swell at the distal extremity. 

 forming a set of club-shaped spokes round the main tubercle ; these are 

 little by little separated from it, and become independent elliptical tubercles 

 at first, and then miliaries or secondary tubercles (PL VII. f. .'. 8, ll, 15). The 

 ten large buccal plates of the actinal membrane are the first to appear, as 

 in Strongylocentrotus (PL IX. f. 2, 4), Toxopneustes (PL VII. f. 7), and 

 Temnechinus (PL VIII. f. 17). Small plates (in genera in which they are 



