BIHANG TILL K. SV. VET.-AKAD. HANDL. BAND 18. AFl). IV. N:0 1. 25 



mordial persistent plates of its ambiilacra arranged in a eonti- 

 inious circle round the future mouth, PJ. IV, fi;/. 25, 2(i. But, 

 whereas in the Cidarid eaeh of them becomes overlapped by 

 one of the second circle transformed into a scale, and tliis si- 

 inilarly ])y a third and so on. and a flexible snccession of pe- 

 (licel-bearing scaly ambulacrals thus is effected from the month 

 npwards, we have seen that in the Echinid, already before 

 the close of the astomous stage, the primordial circle has con- 

 tracted relatively to the circumference of the peristome, Pl. 

 V, fig. 29, and that there has intervened between them a cir- 

 cnlar belt of flexible membrane, in which are contained as- 

 semblages of calcified corpnscles of irregnlar form, destitute of 

 pedicels. The original sequence of pedicelligerous plates is 

 thns broken between the fixed ambnlacrals and the primordial. 

 This interruption is due, not only to the progressive widening 

 (Inring growth, bnt. also, and perhaps chiefly, to the reabsorp- 

 tion of calcareons tissue which from an early period is at work 

 at the adoral margin of the flxed corona. It has been seen 

 that while the primordial pairs of ambulacral plates remain 

 persistent throngh life, the second pairs, together with the so- 

 litary primordial interradials, are very soon worn away, Fl. 

 IV, fig. 25, 26, 28, Fl. V. fig. 29, 30, not in a manner con- 

 sistent with the immanent mode of formation of each plate, 

 bnt as if throngh an agency from Avithout, emanating from 

 the oral centrum, and aflecting little by little the whole of 

 their several adoral margins with a uniformity all around 

 which in this group becomes most obvionsh' manifest in the 

 strict circiilarity of the peristome. In tlie Strongylocentrus 

 dröbakensis O. F. M.^ this abrasion goes on in sncli a manner 

 that the pedicellar pore marked 2 which in a specimen of 8 

 mm. 2 has its place near the lateral n.iddle of its plate, in a 

 specimen of 6 mm., and in others a little older, is found close 

 by the adoral margin and there partly absorbed, and that 

 it soon is followed by the pore marked o; that the spherid 

 and the verrnca, which in the 3 mm. specimen-' are the first 

 in III a, are löst in the (i mm. specimen,* as is also the second 

 spherid, III h, in the specimen of lö mm.^ It is thns seen 



' See Etudes s. 1. Ecliinoulées. p. -JO. •J4. .'37, Pl. XVII. tis?. 140—147. 



'- 1. c. %. 141—144. 



' 1. c. lig. 140, 141. 



" 1. c. tig. 144! 



« 1. c. tig. 14(;. 



