82 THE VOYAGE OF H.M.S. CHALLENGER. 



ambulacra well above the level of the arm-grooves ; and there are uo 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 nohilis (PI. XLI. 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 pro.ximal 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 (PI. XLI. fig. 12; PI. 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 Pentacnnns asteria 

 (PL XIII. fig. 16; PL XLI. figs. 4, 12, 13 ; PL XLVIL fig. 13 ; PL LL 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 iU 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 

 Comatulse (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 nodosits (PL XLI. 

 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 joints 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 XLVII. 

 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 (PL 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 j^roximal parts of the ambulacra. 



Although there are no side plates on the arms and pinnule-bases of Hijocrinus, yet 

 they are large and well developed on the enlarged portions of the pinnules which contain 

 the genital glands (PL Vc. 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 disapj)ear altogether, so that the covering plates 

 come to rest directly on the edges of the pinnule-joints (PL Vc. figs. 8, 9). 



