134 THE VOYAGE OF H.M.S. CHALLENGER. 



and they often alternate more or less regularly on opposite sides of the medio-ventral 

 line of the pinnule, where there is an opening in one of them for the exit of the genital 

 products. 



In the young individuals obtained, even in those with a spread of 12 cm., there 

 is no trace of the enlargement either of the pinnule-joints or of the protecting 

 plates (PI. XVI. fig. 5), although both are visible in the older forms, which still show a 

 considerable part of the first radials externally. In the regenerated arms too the lower 

 pinnules are for some time quite small and inconspicuous, and altogether different from 

 those of the uninjured mature individuals. This is the case even when the arm 

 has attained almost its full size, and is absolutely larger than those of other individuals 

 not yet quite mature, but with comparatively large genital glands. 



All these greatly enlarged genital pinnules are devoid of ambulacra, like the non- 

 tentaculiferous posterior arms of Actinometra ; but at about the position of the twenty- 

 fifth brachial there is a sudden diminution in size both of the pinnule-joints and of 

 the protecting plates, more especially of the latter. They become much smaller 

 and relatively more numerous, w T hile the sacculi which are absent in the large lower 

 pinnules begin to appear, just as they show themselves in the genital pinnules of 

 Antedon angusticalyx from the same station ;* while eventually the ambulacral skeleton 

 shows itself above the small protecting plates, as in Antedon incerta. 2 A little further 

 out on the arms these protecting plates disappear, and the ambulacral skeleton comes to 

 rest directly upon the pinnule-joints, as shown in PL XVI. fig. 4. The side plates are 

 very well differentiated and are often notched for the recejttion of the sacculi or of 

 portions of them ; but in other cases, when the sacculi are large, they are altogether 

 covered and concealed by the side plates. 



2. Antedon discoidea, n. sp. (PI. X. figs. 1, 2). 



Specific formida — A. — . 



Centro-dorsal a thick disk, bearing fifteen to eighteen cirri in a single or partially 

 double row, with the dorsal surface free. The cirri reach 27 mm. in length and consist 

 of forty or fifty joints, a few of which at the base are longer than wide, and the following 

 ones gradually develop a sharp dorsal keel. 



Three radials visible ; the first short, except at the angles of the calyx, where the 

 ends of the basal rays sometimes appear. Second radials short, wide and oblong, and 

 the axillaries barely pentagonal. 3 Both joints have large muscle-plates, and their dorsal 

 surfaces rise towards the middle of their apposed edges. Pays well separated. 



1 See Part I., pi. liv. fig. 5. 2 See Part. I., pi. liv. fig. 6, 



3 The above description applies to the joints as seen in Ml face view. They have a very different shape in the 

 figure of the entire animal, owing to the angle at which the rays are set on the calyx (PI. X. fig. 1). 



