(368 PEDICELLARISE. 



6, 7). In one of the Echini discovered by M. Pourtales, the fixed spines 

 cover the whole upper part of the test (PL IV. f. 8, 10, 12, !■:)■ the movable 

 spines being limited to a circumscribed area along the edge of the shell (Po- 

 docidaris). 



If we trace the development of the spines of starfishes, we find something 

 similar ; but as the pedicellarise are clustered round the base of the longer 

 spines, we are able to distinguish in the earliest stages what will become a 

 spine and what will eventually form pedicellariaa, — a distinction which it is 

 not possible to make in Echini, where the pedicellariaj and spines are irregu- 

 larly scattered. This is especially the case in such genera as Arbacia and 

 the like, in which there are so-called embryonal spines remaining always 

 fixed immovably to the test (PI. XXVI. f. 6, 7). 



In our common starfish I have traced the earliest stages of the spines and 

 pedicellarise (Fig. 11), and have found that at first it is impossible to distin- 

 guish between a spine and a pedicellaria ; it is only in somewhat later stages 

 that the first trace of a difference can be detected (Fig. 12) ; subsequently 



Fig. 12. Fig. 13. Fio. 14. Fio. 15. 



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there is no doubt whatever, owing to the greater and more rapid develop- 

 ment of the central spine, as to what will form spines or pedicellarise 

 (Fig. i.i). In one of the pentagonal starfishes of our coast (Hippasteria) it is 

 even easier to trace the gradual passage of the original limestone network, 

 either, on the one hand, into a spine, or, on the other, into bipartite pedicel- 

 larise. In Fig. n we can easily trace the development of a simple central 

 granule, surrounded by smaller granules, into a short spine ; or by the split- 

 ting of the granule we have gradually formed a slight furrow, then a deeper 

 groove, till two clappers are formed (Fig. 15), which eventually become mov- 

 able, and act as pedicellarise, though they are the simplest forms of that organ. 

 In another starfish, the genus Luidia, the central granule, surrounded by 

 smaller granules, develops either into a spine which passes through the stages 



