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 Comatulee, like Pentacrinus naresianus (Pl. XXXa. figs. 1, 10a, 10), 
12a, 12b), have a syzygy in the third 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 Comatule. For it is only in 
a very few species like Actinoietra fimbriata and Actinometra multiradiata 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 Comatule. 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 0 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 Pentacrinide, however, this is only the case to a very slight extent. The 
syzygial interval is perhaps most regular in Pentacrinus naresianus (Pl. XXVIIL); but 
it is long as in many tropical Comatulz, and in other Pentacrinide the brachial syzygies 
are usually “ few and far between.” 
In Rluzocrinus 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 
unions alternate with one another continuously from the calyx to the arm-ends (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. 
i 
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