PEDICELLARI.E. 



669 



of Fig. 16, and terminates in a long slender spine, surrounded by papillae at 

 its base ; or the central spine of Fig. ie is, like the central granule of Hip- 

 pasteria, little by little split into three, and finally forms a passage through 

 such forms as are given in Fig. is, into short tripartite pedicellariae, sur- 

 rounded by isolated spines at the base. 



If anything further were required to prove the homology between spines 

 and pedicellariae, it is the case of tripartite, pedunculated, Echini pedicellaria? 

 attached, as common spines are, upon a tubercle, surrounded by the pecu- 

 liar smooth area called the scrobicular circle ; this form of pedicellariae is 

 actually found in the genus Podocidaris (PI. IV. f. 1,3). The same reason- 

 ing will readily suggest to the student of Echinoderms the homologv of the 



Fig. 16. 



Fig. 17 



Fig. 18. 



so-called claws of Ophiurans (Fig. 17) and of the anchors of Holothmians 

 ( Fig. is), which, although used for such totally different functions, — a sort of 

 prehensile organ, for motion along the ground. — are in reality in their turn 

 only modified spines, or different forms of pedicellariae 



Although the spines of our sea-urchin are apparently so different from 

 the pedicellariae, yet when we pass in review the whole order of Echini 

 we find differences among the spines fully as great as those observed in the 

 pedicellariae. What can be more diverse than the immense, slender, hollow 

 spine of an Echinothrix (PL III"./. 1), or of a Diadeina, six to eight times the 

 diameter of the test, and the short flattened spine, forming a regular pave- 

 ment on the test of Colobocentrotus (PI III d . f. 4) ; we find such extremes 

 as the club-shaped, curved ambulacral spines of Salenia (PI. Ill f. 8) ; the 

 papillae of Cidaris ; the sharp, solid, curved antennae-like spines of Coelopleu- 

 rus (PI. IP. f. 14); the massive bat-shaped spines of Heterocentrotus (Pi. 

 III d . f. e ; PI IIP) ; the cupuliform spines of Goniocidaris (PL P.f. 3, 4) ; 

 the slender silk-like spines of the Clypeastroids (PL XIII". f. 4 ; PL XIIP. 



