82 THE VOYAGE OF H.M.S. CHALLENGER. 
ambulacra well above the level of the arm-grooves; and there are no other plates 
on the arm than the covering plates which really belong to the pinnule-bases, while 
the muscular bundles are freely visible at the sides of the ambulacra. But in other species, 
such as Metacrinus nobilis (Pl. XLL fig. 11), the food-groove is more concealed within 
the arm-groove, and the forked covering plates are less abundant at the pinnule-bases. 
Farther out on the pinnule, the proximal half of the fork becomes gradually less and 
less prominent ; and it is eventually absorbed into the basal part of the plate, which thus 
represents a side plate; while the distal half of the fork, becoming larger and better 
defined, separates itself off as a rounded covering plate (Pl. XLI. fig. 12; Pl. XLVII. fig. 11). 
The branches of the ambulacra which pass on to the massive basal joints of the 
prismatic lower pinnules are usually but little plated, as is the case in Pentacrinus asteria 
(Pl. XIII. fig. 16; Pl. XLI. figs. 4, 12,13; Pl XLVIL fig. 13; Pl LI fig. 12). But 
beyond the first two joints the plating reappears; and the four rows of plates become 
gradually developed from the irregular plates at the sides of the groove, which come to 
assume a definite form and break up into covering plates and ill defined side plates. 
The gradual differentiation of side and covering plates upon the pinnules from the 
single forked plates at the sides of the brachial ambulacra takes place in this way in most 
species of Metacrinus; but the four rows are never so distinctly separable as in the 
Comatulee (Pl. LIV. figs. 4, 6-9). 
A slight variation of this process occurs in Metacrinus costatus (Pl. XLVII. fig. 13; 
Pl. XLIX. figs. 6,7); while Metacrinus murrayi and Metacrinus nodosus (Pl. XLL 
fig. 12; Pl. LI. fig. 12) are the intermediate links between this species and the other types of 
Metacrinus. The bases of the pinnule-ambulacra just beyond the wide lower joimts are 
bordered by a series of rounded plates, which are deeply hollowed in the centre so that 
their edges stand up rather prominently. The first eight or ten of these are attached to 
the pinnule-joints on each side by a continuous band of limestone. This gradually becomes 
absorbed into the raised proximal edges of the rounded plates so as to form the side plates; 
while the distal halves eventually separate themselves off as the covering plates (Pl. XLVI. 
fig. 13). The side plates only become properly differentiated in the outer parts of the 
lower pinnules, and in the later pinnules on the arms (PI. XLIX. figs. 6, 7); but they 
retain a more or less prominent backward process, which is the remains of the raised 
hinder edge of the rounded plates on the proximal parts of the ambulacra. 
Although there are no side plates on the arms and pinnule-bases of Hyocrinus, yet 
they are large and well developed on the enlarged portions of the pinnules which contain 
the genital glands (Pl. Ve. fig. 10, sp). The proximal ones, taking the place of 
numerous small anambulacral plates, are smaller than their successors, which considerably 
increase the depth of the body-cavity within the pinnule. Distally, the side plates 
gradually diminish in size and finally disappear altogether, so that the covering plates 
come to rest directly on the edges of the pinnule-joints (PI. Ve. figs. 8, 9). 
