BIHANG TILL K. SV. VET.-AKAU. HANDL. BAND 13. AVD. IV. N:0 O. 91 



is left, are replaced by tlie growtli of othcr forms, large and 

 separate near the tubercles, minute and densely packed on 

 thc disk, Tab. 7, fig. i, 3, 4. 



The Tal). $, p<j. 7 represcnts the juvenile appearance of 

 these parts in an Arbacia stellata Blainv., 6 mm. in diameter. 

 The dnll colour of the plates and the tubercular system sets 

 ofF the bright red of the epistroma. The sexual pores are 

 still missing, but the radiating ridges of the eostals are largely 

 multiplied, densely packed, discontinued över the siitures, and 

 preparing to break up into minute nodules. On the ambulacra 

 the glossy ridges are still intact and on each of them five or 

 six of the berry-like protuberances stånd upright, the lowest 

 of them approachcd by thc uppermost tubercles in the act of 

 risino-. In the interradia the alteration is more advanced, the 

 rival structures meet much sooner, already on the third and 

 fourth plate from the top. The strong glossy ridges are 

 effaced, remoulded and converted into numerous dense rows 

 of minute nodules, most conspicuous and brightly coloured 

 where the protuberances have stood, near the middle suture. 

 On the upper three or four plates the protuberances are still 

 left, but reduced, and lower down, where they are neared 

 more and more by thc advancc of nascent tubercles, they sink 

 down, diminish and dissolve, until their only remains are little 

 masses of minute nodules of a bright red colour sticking to 

 the cone upou its aboral sidc, Tab. B, fig. 8, or even nothing 

 but a reddish tint left on a part of the thin film of the cu- 

 ticula. 



Another specimen, a little larger, 7,5 mm. in diameter, 

 is more changed, but still retains much of the epistroma in 

 its early form. On the eostals, now provided witli sexual 

 pores, it is alreadv reduced and transformed into a thin, com- 

 pact, bright-red layer of divaricatingly radiating very minute 

 nodules. The radials still bcar their threci protuberances, and 

 from the projecting mesial septum thc two ridges descend 

 on the sides of the ambulacral areola, presenting at regular 

 intervals a series of strong protuberances, up to eight in a 

 row, until, a little above the equator, the tubercules are en- 

 countered. lu the interradia this comcs in the third plate 

 from the calyx, and evcrywhere the epistromal glomeration is 

 seen to lose form and to spread upon the cone of the rising 

 tubercle. Further on, as the animal grows, the epistroma gra- 



