ANTEDON. 381 



ventrolateral ones lodge the inter-articular ligaments ; 

 while the ventral pair lodge the flexor muscles. Actual 

 contact between the successive segments of the arm occurs 

 only along the great transverse ridges. The distal 

 articular face of the first secundibrachial and the proximal 

 articular face of the second resemble the proximal face of 

 the second primibrachial and the distal face of the first in 

 having a vertical ridge separating a single pair of lateral 

 fossae. 



Exceptions to this general rule are presented by the 

 distal faces of certain segments and the proximal faces 

 of the next succeeding ones, which are almost flat, 

 and the articular and muscular fossae are replaced by 

 a series of slightly elevated ridges and alternating furrows, 

 which radiate from the opening of the axial canal to the 

 dorsal and lateral margins (PI. I., fig. 16). Ligamentous 

 fibres only (PL Y., fig. 50) bind the two apposed faces 

 together. To this close and immovable union of two 

 segments, the direction of which is always at right angles 

 to the axis of the arm, the name syzygy was given by 

 Joh. Miiller (PI. I., fig. 3, syz.). Of the two segments 

 concerned in the formation of a syzygy only the distal one 

 (epizygal) bears a pinnule, the proximal (hypozygal) 

 never has one. In Antedon bifida syzygies occur between 

 joints 3 and 4, 9 and 10, 14 and 15, and then between 

 every fourth and fifth or sixth and seventh. Syzygies 

 appear to be points of least resistance, at which autotomy 

 very frequently occurs. 



A transverse interbrachial muscle runs from arm to 

 arm of each pair, and its two ends are lodged in corre- 

 sponding rounded fossae excavated in the inner ends of the 

 proximal faces of the first primibrachials (PI. I., fig. 12). 

 The pinnules, like the arms, are composed of 

 articulated segments, the diameter of which gradually 



