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Show 734 MR. P. H. CARPENTER ON THE [Dec. 1 9, joints which become immovably united and behave in all respects as one. But it mav be united by syzygy to the second radial, instead of by the usual bifascial articulation1. This, however, is of an altogether different morphological value from the syzygial unions of the arm-joints. In the latter case the hypozygal entirely loses its individuality as a separate joint, and bears no pinnule as the epizygal and the'remaining brachials do. Thus, for example, in very nearly all Comatula 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"2. 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 Ant. rosacea, for example, the 3rd and 4th, the 9th and 10th, and the 14th and 15th joints of the growing arm are respectively united in pairs by syzygy; but the arm is best described as having syzygies in the 3rd, 8th, and 12th joints. So again in the numerous Comatula, such as Act. parvicirra, which have axillaries on some or all of the primary arms. Counting from the third radial, the distichal axillary is primitively the fourth joint. The first, as is almost invariably the case, bears no pinnule, while the second does bear a pinnule, but the third 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) being a syzygy. In Encrinus, in most recent species of Pentacrinus, in a few Coma-tnlse (Act. Solaris, &c.) the two outer radials and the first two joints beyond them are respectively united by syzygy ; and on the principle explained above, each pair would be considered as forming a single joint, so that the true third brachial (itself a syzygial joint) would come to be the second. This would involve our describing these forms as having but two radials, the axillary with a syzygy, and syzygies both in the first and in the second brachials. I think, however, that this would be misleading and make the difference between the two types appear much greater than it really is. The presence of three radials is such an absolutely constant character in all the five-rayed Neocrinoids, excepting Metacrinus & Plicatocrinus, that the fact of the two outer ones being united by syzygy and not articulated seems to me to be of minor importance ; and I do not assign to it the same morphological value as the syzygial union of the third and fourth primitive brachials, in which 1 I use this name to denote the presence of a ligamentous bundle on either side of a vertical articular ridge, while muscles are absent. In by far the greater number of Comatulce this is the mode of union of the two outer radials and of the first two joints beyond each axillary. It also occurs in Millericrinus Apiocrinus, Pentacrinus decorus, P. blakei and other types. But in Bathycrinus there is a trifascial articulation, a small additional bundle of fibres being inserted into the apposed faces of the joints at the lower or dorsal end of the articular ridge. 2 W . B. Carpenter, Phil. Trans. 1866, p. 721. |
