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 palseozoic times, 
and 1 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 
radioles 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. 355. — Temnechi- vig, Ou) : Fig. 350. — Trigonocidaris 
= lat. 2,5 The Arbaciade, a albida. T 2, 
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 Coelo- 
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. 357. — Podocidaris 
sculpta. 1;0, 
