92 



THREE CRUISES OF THE " BLAKE. 



ment are plainly to be seen in the unequal development in the 

 size of the genital and ocular plates throughout the group of 

 echini. Perhaps we may trace the differences in the develop- 

 ment of the ambulacral and interambulacral zones in the echini 

 to such a primitive differentiation. This embryonic feature runs 

 back through the echinoid series of the earlier paleozoic times, 

 and I am inclined to look upon the suranal plate of Salenia as 

 recalling the crinoidal affinities of the sea-urchins, though it has 

 not taken in the development of these the important part which 

 it occupies in the starfishes and crinoids. The spiny primary 

 radicles of the large specimens are formed from the gradual wear- 

 ing of the delicate filaments (Fig. 354) of the corresponding 

 spines in younger specimens. 



As representatives of the sculptured echini so common dur- 

 ing the tertiaries, and 

 still prominent in the 

 Indo-Pacific fauna, we 

 find the small Tem- 

 nechinus (Fig. 355) 

 and Trigonocidaris. 

 (Fig. 356.) 



The Arbaciadse, a 

 family of sea-urchins 

 eminently characteristic of the American fauna, both Atlantic 

 and Pacific, are represented in deep water by a highly sculp- 

 tured genus, Podocidaris (Fig. 357), 

 with primary spines recalling the em- 

 bryonic ones of the littoral species. The 

 large spines of these genera are used 

 for locomotion, and for protection are 

 tipped with a sort of shoe, which is con- 

 stantly replaced as it wears. This shoe 

 takes an immense development in Crelo- 

 pleurus (Fig. 358), and grows to three 

 or five times the length of the spine it- 

 self. The primary spines are also curved, and when the urchin 

 is in motion it is raised far above the surface, literally walking 

 on stilts. The deep-water species must by means of their spines 



Fig. 355. Temnechi- 

 nus maculatus. 2 ,-^. 



Fig. 356. Trigonocidaris 

 albida. f. 



Fig. 357. Podocidaris 

 sculpta. *j 8 . 



