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



radials are united by syzygy, as are tlie first two joints beyond tlie distichal and all 

 subsequent axillaries. Except in these and a few other cases, however, there is a very 

 great uniformity throughout the arm-divisions of the Comatulse. 



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 paii-s (PI. XII. figs. 18, 21 ; PI. XV. figs. 1, 2; PL XVI. fig. 1 ; PI. XVIII. 

 figs. 1-3, 8, 11 ; PL XIX. figs. 1, 6, 7 ; PL XXI. figs. Id, 2d, 5a ; PL XXV.; PL XXVI. 

 figs. 4, 5, 8). On the other hand, the ten-armed Pentaa-inus naresiamis 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 hIaJcei 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 ; Pis. 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, 23; PL XXXa. figs. 10«, 106, 12a, 126; 

 PL XXXII. fig. 4 ; PL L. figs. 6-16). Thus, for example, in very nearly aU Comatulse 

 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 joints 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. So again in the numerous Comatulaj, such as Actinometrcc parvicirra, 

 which have axUlaries on some or all of the primary arms. Counted from the third radial, 

 the distichal axillary is primitively the fomth joint. The first, as is almost invariably 

 the case, bears no pinnule, while the second has a pinnule, but the thuxl not, for it is 

 united to the following (axillary) joint by a syzygy. 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. Tram., 18C6, p. 721. 



