THE CRINOIDEA 



FIG. XXII. 



Evolution from uniserial, througli zigzag, 

 to biserial bracldals. 



of food. The axial cords and ventral groove at first swing from 

 side to side ; but this would be almost impossible in biserial arms, 

 so here a common straight ventral groove is formed, and the axial 

 cord lies at the bottom of it. The change from uniserial to 



biserial, just as the evohation 

 of pinnules, begins in both 

 ontogeny and phylogeny at 

 the growing tip of the arm, 

 and proceeds gradually proxi- 

 malwards (Fig. XXII.). 



Still further develop- 

 ment occurs in the Camer- 

 ata. The adjacent right and 

 left ossicles of a biserial arm 

 may fuse, so as to form a compound brachial, and this necessarily 

 bears two pinnules, one on either side. Further than this, it appears 

 as though two or more compound brachials could fuse, and so form 

 a triply compound ossicle bearing two or three pinnules on either 

 side. At the same time, the pinnulars themselves may come to lie 

 in zigzag or biserial fashion, in the same way as do the ossicles of 

 so many cystid brachioles. It is this structure that influenced 

 Jaekel in his distinction between "pinnulae" and "ramuli" (supra) ; 

 but the facts are explicable as the final stage in a regular 

 evolution (Fig. LXXIX.). 



Fusion of brachials either laterally, or in vertical series, or both, 

 may occur in any crinoid race in which it proves advantageous. 

 In some Gissocrini the IIBr, and possibly IIIBr, of each arm were 

 laterally united by suture ; in Crotalocrinus (Fig. XCII.) all brachials 

 of an arm are suturally united by projections at the distal margin 

 of each brachial ; in Petalocrinus (Fig. XCI.) all brachials of an arm 

 except IBr are fused into a single petaloid plate. Compare also 

 Melocrinus (Fig. LXXIV.) and Eudadocrinus (Fig. LXXI. 4). 



Brachials primitively, and pinnulars nearly always, are 

 united by loose suture (compare Fig. XVIII.). The next stage is 

 imperforate articulation. In the final stage, perforate articulation 

 (Fig. XXIII. 1), there is a well-marked transverse fulcral ridge, 

 pierced by the axial canal ; the ventral groove comes nearly up to 

 the ridge at this point. On each side of the ventral groove, and 

 often separated by a slight vertical, i.e. dorso-ventral, ridge run- 

 ning down to the axial canal, are two depressions, fossae ; the ventral 

 pair lodges muscle fibres (" muscular fossae ") ; and the dorsal 

 pair, interarticular ligament (" ligamentar fossae "). Dorsad of 

 the fulcrum is a deep "dorsal fossa," lodging elastic ligament. 

 This type may be modified by the disappearance of the ventral 

 muscles, the increase of the interarticular ligaments and their 

 fossae, and of the vertical ridge separating them, which now 



