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DEVELOPMENT OF ANTEDON (COMATULA, LA3IK.) EOSACEUS. 721 



50. Fourth Brachial. — This segment (Plate XXXVI. fig. 7) has the same general 

 form with the preceding, and the obliquity of its two faces (c, d) is in the same direction, 

 the longer being again the internal. While the distal face of this segment (b) conforms 

 to the ordinary type, and is distinguished from the proximal face of the Fourth segment 

 (with which it might otherwise be confounded) by possessing an articular socket for the 

 pinnule {y, figs, b, c, e), its proximal face (a) shows the same ridge and furrow arrange- 

 ment as the distal face of the Third. — When we examine the Third and Fourth segments 

 in theii- natural apposition (fig. 6, c, d, e), we see that the two sets of ridges are applied 

 to each other {sg), leaving between them flattened passages that are formed by the cor- 

 respondence of the furrows. The adhesion between these apposed surfaces is so close, 

 that it can only be dissolved (like that of the First Radials to each other and to the 

 Centro-dorsal plate, §§ 31, 32) by boiling in a solution of caustic alkali. No ligamentous 

 substance is interposed between them ; but an examination of decalcified specimens shows 

 that the canals are occupied by radial extensions of the ordinary sarcodic basis-substance 

 (Plate XLIII. fig. 2). The peculiar arrangement of these suggests that, like the " medul- 

 lary rays " of an Exogenous Stem, they may serve to establish a communication between 

 the "medullary axis" of this basis-substance which occupies the central canal, and the 

 " cortical envelope" by which the surface of the segment is invested. When we come to 

 study the development of the Brachial segments (§67), we shall find that the syzygies 

 do not originate (as has been supposed by some) in an imperfect subdivision of segments, 

 — no subdivision, perfect or imperfect, ever taking place ; but that they are formed by 

 a partial coalescence of segments originally quite distinct. For in the early stage of the 

 existence of this animal as a detached Antedon, there is still so little specialization in 

 the rod-like segments of the arms, that they are all nearly similar in form, have no 

 proper articular surfaces, and are held together by nothing else than an imperfectly 

 fibrous sarcodic substance. And whilst the majority of these gradually come to possess 

 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. These syzygies are repeated at more or less frequent 

 intervals along the greater part of the length of the arms ; and as they are normally 

 pretty constant in position in the several Arms of each individual, and in the several 

 individuals of one species, whilst usually diverse in those of different species, it has been 

 proposed by Professor J. Mullee to use them as characters of specific definition. To 

 this it has been objected by Dujardin' that the syzygies are really variable in their 

 number and in their mode of distribution on the several arms, especially when there has 

 been a reparation after injury, so that on the same individual there may intervene from 

 four to nine ordinary articulations between the corresponding syzygies. Whilst fuUy 

 admitting the existence of irregularities thus originating, which are for the most part 

 easily recognized, 1 am disposed to believe with Muller that there is a normal type 

 which is constant — as regards the basal portions of the arms at least — for each species, 

 ' Histoire Naturelle des Zoophytes Echinodennes, p. 193. 



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