732 OJS THE fOUNG STAGES OF ECHINI. 



In Strongylocentrotus Drbbachiensis, soon after resorption of the pluteus 

 the young sea-urchin has few large primary tubercles, limited to the ambi- 

 tus, Podocidaris and Colobocentrotus-like (PL IX. f. 1). The next stage 

 has two principal rows of large tubercles, — occupying the whole test 

 (PL IX. f. .;), Cidaris-like, no miliaries, — increasing in number as they grow 

 older, the spines gradually passing from a condition similar to those of 

 Rhabdocidaris (PI. X.f. .'-.'/), Cidaris, Ai'bacia, and finally to Strongylocen- 

 trotus-like spines ( PI X.f. l), as fast as the primary tubercles are formed, 

 retaining their embryonic features most strongly while the spines are di- 

 rectly connected to the test (PI. X. f. 2-4), as the}' are in Podocidaris. In 

 the earlier stages the actinal opening is large (Arbacia-like), without inden- 

 tations (PI. IX. f. 2-.',). Cidaris-like, occupying nearly the whole of the 

 actinal surface. As the test increases this opening becomes proportionally 

 smaller, and slight cuts are formed (PI. IX. f. to), Echinus-like. The anal 

 system is at first closed by a single subanal plate (PL X.f. :). appearing be- 

 fore the formation of the genital and ocular plates ; it remains for a consid- 

 erable period more prominent than the other plates, which are gradually 

 added to cover the enlarged anal system (PI. IX. f. 8, 6, 7). The symmet- 

 rical axis of the subanal plate does not hold a fixed relation to the niadreporic 

 body, being opposite different genital plates in various stages of growth. 

 This corresponds to the oblique position of the subanal plate in Salenidae 

 ( /'/. ///./'. //), when we take as starting-point the niadreporic body. The 

 abactinal system subsequently passes through a stage (PL IX. f. 11) re- 

 minding us of Arbacia (PL V.J'. .') and Trigonocidaris (PL IV. f. ;), only 

 there are five instead of four anal plates. The poriferous zone is at first 

 narrow, the pores arranged in vertical rows; subsequently they are slightly 

 arched vertically ( PI. IX. f. :,); they next separate into horizontal arcs made 

 up of a smaller number of pores, increasing rapidly in number with age 

 (PL IX. f 9); in small specimens we can trace their mode of formation, as 

 the arcs near the ambitus are similar to those of the adult, while those next 

 the abactinal system are similar to those of the younger stages. The plates 

 of the poriferous zone increase independently of the interambulacral plates. 

 The different stages of growth represent in the younger stages combinations 

 of features strongly recalling at first Cidaris (PL IX. f. 2), next Hemicidaris, 

 then Pseudodiadema, Arbacia, and Stomopneustes. The same general changes 

 take place in Strongylocentrotus lividus; the turban shape (Cidaris state) of 

 the young test is even more striking than in Strongylocentrotus Drobachiensis. 



