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



three radials ever has a pinnule on the second one; and when this becomes the hypozygal 

 of a syzygy, it does not therefore lose its individuality, as is the case with the hypozygals 

 of the ordinary brachial syzygies. Almost the same may be said respecting the first two 

 brachials. Most Comatulse, like Pentacrimis naresianus (PL XXXa. figs. 1, 10a, 106, 

 12a, 126), have a syzygy in the thiixl brachial with a bifascial articulation between the 

 two preceding joints, of which only the second bears a pinnule. Hence, when these 

 two are united by syzygy, as in Actinometra Solaris, Actinometra typica, &c., the lowest 

 or hypozygal loses no individuality as an arm-joint. They are, therefore, better 

 described as the first and second brachials, and not as a first brachial which "is a syzygy." 

 This method has the advantage of retaining the third brachial as a syzygial joint as a 

 condition which is common to by far the larger number of Comatulse. For it is only in 

 a very few species like Actinometra Jimbriata and Actinometra multirad lata that there is 

 a syzygy in the second brachial and a pinnule on the first, as is often the case in 

 Metacrinus. This is an entirely different type, and arises from the coalescence of the 

 primitive second and third joints of the growing arm. 



Syzygial unions of two primitively separate arm-joints occur with great regularity 

 throughout the arms of the Comatulse. In the two principal genera Antedon and 

 Actinometra, there are large groups of species typified by Antedon eschrichti and 

 Actinometra parvicirra respectively, in which syzygies occur at tolerably regular inter- 

 vals of three joints. It is rare, however, to find a perfectly regular arm, especially in. 

 the latter species, in which the "syzygial interval" may vary from to 10 joints.^ In 

 other species the interval may be as much as twenty joints or more ; while it is occasion- 

 ally two, as in Antedon rosacea, and in rare cases one joint only, as in Rhizocrinus. But 

 it is generally possible to find a considerable amount of regularity in the number of joints 

 which form the syzygial interval in any given species, and this is often of some value for 

 systematic purposes. 



Among the Pentacrinidge, however, this is only the case to a very slight extent. The 

 syzygial interval is perhaps most regular in Pentacrinus naresianus (PI. XXVIII.) ; but 

 it is long as in many tropical Comatula^, and in other Pentacrinidce the brachial syzygies 

 are usually " few and far between." 



In Rhizocrinus and Hyocrinus, on the other hand, the syzygial union of the primitive 

 brachials is carried on to a very great extent. In the former genus syzygial and muscular 

 iinions alternate with one another continuously from the calyx to the arm-cuds (PL IX.; 

 PL X. fig. 20 ; PL LIII. fig. 7). In Hyocrinus (PL VI. figs. 1, 2), as was well described 

 by Sir Wyville Thomson,^ the five arms "consist of long cylindrical joints deeply grooved 

 within, and intersected by syzygial junctions. The first three joints in each arm consist 

 each of two parts separated by a syzygy ; the third joint bears at its distal end an 

 articulating facet from which a pinnule springs. The fourth arm-joint is intersected by 



1 Actinometra, loc. cit, p. 49. 2 Journ. Linn. Soc. Lond. (Zool.), vol. xiii. p. 52. 



