50 THE VOYAGE OF H.M.S. CHALLENGER. 
radials are united by syzygy, as are the first two joints beyond the distichal and all 
subsequent axillaries. Exeept im these and a few other cases, however, there is a very 
great uniformity throughout the arm-divisions of the Comatule. 
In five of the eight recent species of Pentacrinus the two outer radials form a 
syzygy, and in correspondence with this the lowest distichals and brachials are similarly 
united in pairs (Pl. XII. figs. 18, 21; Pl. XV. figs. 1,2; Pl. XVI. fig. 1; Pl. XVIII. 
figs. 1-3, 8,11; Pl. XTX. figs, 1,16; 75 Pl. XXII. figs. 1d, 2d, 50; Pl. XXV.; PL XXVI 
figs. 4, 5, 8). On the other hand, the ten-armed Pentacrinus naresianus has a bifascial 
articulation between the two outer radials, and also between the two lowest brachials, 
just as in Antedon rosacea (Pl. XXX. figs. 1,11, 12, 16,17). But in Pentacrinus 
decorus and Pentacrinus blake: the rays divide twice or thrice; and though the two 
first joints beyond the lowest axillaries resemble the outer radials in being articulated by 
ligaments, yet there is a muscular joint between the two lower brachials of the ultimate 
arms, the second of which is usually a syzygy (Pl. XXXI. figs. 1, 2; Pl. XXXII. 
figs. 16-18; Pl. XXXIV. figs. 3,6; Pls. XXXV.-XXXVIL). 
The syzygial union of two arm-joints is of a somewhat peculiar character. For 
the hypozygal entirely loses its individuality as a separate segment of the arm, and 
bears no pinnule as the epizygal and the remaining brachials do (Pl. XII. fig. 9; 
Pl, XV. fig. 3; Pl XXX. figs. 1, 19, 20; 223) PL XXXa. figs. 10a,10b ita, 1b: 
Pl. XXXII. fig. 4; Pl. L. figs. 6-16). Thus, for example, in very nearly all Comatule 
the original third and fourth joints of the growing arm differ from those which ultimately 
appear beyond them. For ‘‘ whilst the majority of these gradually come to possess the 
true articulations, and to be separated by the intervention of muscles and ligaments, a 
certain small proportion become more intimately united on a simpler plan, which admits 
of no motion between them.”! The double or syzygial joimts thus formed resemble the 
ordinary brachials in bearing but one pinnule, and they are therefore best considered as 
single joints. In Antedon rosacea, for example, the third and fourth, the ninth and 
tenth, and the fourteenth and fifteenth joints of the growing arm are respectively united 
in pairs by syzygy ; but the arm is best described as having syzygies in the third, eighth, 
and twelfth joints. Soagain in the numerous Comatulz, such as Actinometra parvicirra, 
which have axillaries on some or all of the primary arms. Counted from the third radial, 
the distichal axillary is primitively the fourth jomt. The first, as is almost invariably 
the case, bears no pinnule, while the second has a pinnule, but the third not, for it is 
united to the following (axillary) joint by asyzygy. The first ray-division would therefore 
be described as consisting of three distichal joints, the second bearing a pinnule, and the 
third (axillary) a syzygy. 
The same arrangement occurs in the genus Metacrinus, which is distinguished from 
Pentacrinus and from all other Neocrinoids by having, not three radials only, but 
1 Phil. Trans., 1866, p. 721. 
