PEDICELLARISE. 



667 



clustered round the base of the spines of the dorsal surface {Fig. u) ; in some 

 starfishes we also find tripartite pedicellarise as in sea-urchins, only they 

 are usually supported upon a very short stem, or articulate directly from the 

 limestone network of the shell. We find similarly in Echini pedicellarise placed 

 in pits (Goniocidaris) in which the stem is reduced to a minimum ; their func- 

 tion is quite problematical, and their movements are reduced to the mere 

 opening and shutting of the valves. It is from the study of the pedicellarise 

 of starfishes that we have been able to form .some accurate idea of the ho- 

 mologies of these interesting appendages. 



We must now go back to the early history of the growth of spines in em- 

 bryo Echinoderms, to obtain the key of the homologies of pedicellarise. In 

 all young Echinoderms the test, i. e. the upper coating of the arms of a star- 

 fish, the envelope of a Holothurian, the shell of a sea-urchin, is made up of 

 an irregular network of limestone cells (Fig. 10), which makes its appearance 



Fig. 9. 



Fig. 10. 



Fig. 11. 



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'con 





in the early Pluteus stages; with increasing size this network becomes closed 

 at certain points, and sends off upright shanks, which little by little form 

 very irregular fan-shaped spines (PL IX. f. i ; PL X.f. 4); in our common 

 sea-urchins these spines are immovable, forming at that stage part of the test 

 itself. As the spines grow they become more pointed (PL X.f. 4), but are 

 still immovable. In somewhat more advanced stages a slight constriction is 

 formed at the base of the spine (PL X. f. s), and very soon after that, below 

 the constriction, a tubercle is formed upon which the spine is articulated, and 

 is then capable of a certain amount of motion by means of the muscular 

 sheath connecting the base of the spine with the tubercle, which fit by a 

 ball-and-socket joint ; soon the spine appears longitudinally striated, the 

 limestone cells of which it was composed when smaller becoming obliterated 

 by the successive circular layers of the older spine. 



In some sea-urchins (Arbacia) we find spines which never become articu- 

 lated, are always fixed, and remind us, although of very different shape, of 

 the embryonic stage of the spines of our common sea-urchin (Pi XXVI. f. 



