Revision of the Holothurioidea. 343 
In some of the young holothurids, most of the spicules belong 
to tlıe developmental series (Pl. 13, Fig. 8—11), beginning with a 
simple rod from which bars grow out to form first one larger central 
hole (Fig. 8), and then a second (Fig. 10), and at the same time 
one smaller terminal hole at each end (Fig. 9, 10). The average 
size of these plates is 5 ux 72 u. 
Thus in this series a well marked button with 4 holes is formed 
(Fig. 10). These plates average 99 « in length, with a range 
from 68 « to 176 u, and 63 u in diameter, with a range from 33 u 
to 86 «. In some young individuals most of the plates are small 
and with few holes. In the further development of these plates, 
bars may grow across the larger central holes (Fig. 11), or other 
terminal and lateral holes may be formed, to enlarge the plate into 
the typical form. 
In the very young, the spicules may be large, well developed, 
and so crowded that they overlap, or else are imperfectly developed 
and scattered. In either case the distribution is about the same in 
trivium and bivium hence my specimens do not agree with the 
observation of TH£EL, 1886, that the plates are more common in the 
dorsal perisome while rare, or totally absent, in the ventral. 
Often one end of the perforated plate, spinose like that of the 
pedicel, projects from the outer surface of the body-wall. When 
these plates occur in large numbers the skin feels rough to the 
touch. In some cases longitudinal sections through the body-wall 
ofthe celoacal region reveal several heaps of larger perforated plates 
which are not so compact, so extensive, or composed of such large 
plates as the similarly placed heaps of COucumaria japonica. 
Since these heaps are not limited to the radial regions they are 
not to be confused with the vestigeal anal teeth described later. 
Spiceules ofpedicels. — a) Supportingrodsand plates. 
The most frequent form is a somewhat elongated supporting per- 
forated plate (Pl. 13, Fig. 12), with one end rounded, or with several 
larger prongs, while the other is dentate with small spines. The 
holes are larger toward the center and smaller and more crowded 
at the ends, particularly at the spinose end which projeets from 
the surface of the pedicel wall (Pl. 13, Fig. 13). 
In some spicules both ends are equally spinose. Frequently 
the base is curved with the wall of the pedicel. In the adult the 
mean number of holes is 44 with a range from 23 to 86. The mean 
length is 211,5 «, with a range from 162 to 351 u, and the mean 
