ANTEDON 379 



In general plan these latter correspond with the articnlar 

 faces of the radials and first primibrachials, there being 

 a strong transverse ridge that separates a deep fossa on its 

 aboral side from two pairs of fossae on its oral side 

 (fig. 10). The divergent lamellae described above form 

 vertical dividing ridges between the latter. The radial 

 canal, which has been seen above to traverse the first 

 primibrachial, enters the second and immediately bifur- 

 cates, and the oval apertures of its two branches appear on 

 the corresponding oblique articular faces at the point 

 where the transverse ridge is joined by the vertical one. 



Interradial Plates.— In some specimens of Antedon 

 three or four small interradial plates are found in the 

 interradii between the axillaries. 



The Arms. -The arms (PL L, figs. 1, 2 and 3) are 

 rather slender and taper gradually to extreme tenuity. 

 Each arm is composed of a long series of joints or 

 segments (figs. 3 and 21), placed end to end and bound 

 together by muscular and ligamentous fibres (PI. V., 

 fig. 50). These segments are termed secundibrachials, 

 and are of the same fundamental form throughout the 

 length of the arm, each one being a short cylindrical rod, 

 traversed by a minute axial canal (figs. 12 to 16, also 

 fig. 21). The absolute length of the segments decreases 

 very gradually from the base to the tip of the arm, those 

 at the latter extremity being rather more than half the 

 length of those at the former. The proportion of length 

 to diameter presents considerable variation. Thus, at the 

 proximal end of the arm the length of the first few 

 segments is less than half their diameter; in the middle 

 region their length is less than twice their diameter ; while 

 at the distal extremity of the arm the segments become 

 more and more cylindrical in form, and their length is at 

 least four times their diameter. Viewed from the aboral 



