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



to the extremity of the delicate innermost spine. One or two small ciliary spines may 

 be present on the extreme outer edge of the adambulacral plate, adjacent to the first 

 longitudinal row of intermediate plates ; and two or three similar small spines are present 

 in the same position at the outer edge of the non-prominent intermediate adambulacral 

 plates, but no spines whatever are present on the surface of these plates within the 

 furrow. 



The actinostome is deeply depressed, and the mouth-plates are entirely within the 

 cavity, and are not apposable. Their armature consists of pointed, moderately robust 

 spines similar to the larger spines on the ridges of the adambulacral plates. 



The madreporiform body, which is small and inconspicuous, is placed external to one 

 of the interradial (basal) plates. 



The anal aperture is small, distinct, surrounded by a circlet of small ciliary spines, 

 and is placed at the side of the dorso-central plate, and consequently slightly excentric 

 in position. 



The ambulacral tube-feet form four rows. They are rather small, subconical, and 

 have a small but distinct fleshy terminal disk. 



Young Phase. — The j'oung form, 1 measuring R=ll mm., r = 2 - 25 mm., has a very 

 remarkable appearance, owing to the prominence and distinctness of the component parts 

 of the skeleton. The disk is much higher than in the adult. The dorso-central plate is 

 prominent and assumes the shape of a rounded cone. The basal or interradial and first 

 radial plates are of nearly equal size, and are very tumid or almost semiglobular in form. 

 The plates of the median radial line are large and distinct, occupying a large portion of 

 the abactinal surface of the ray. The supero-margiual series of plates form the margin of 

 the ray and the intermediate plates are small. Between the supero-marginal series and 

 the adambulacral plates there are not more than two fully developed longitudinal rows of 

 plates, with a partially developed series commencing to appear between the latter and the 

 adambulacral plates. The terminal (ocular) plates are very large, somewhat resembling 

 the shape of a serpent's head, and are armed with one or two pairs of comparatively large 

 robust spinelets, near the extremity, which are directed upward. 



The large plates of the disk and the median radial series have already a small tubercle, 

 but only some of these bear spinelets. All the plates have a few widely spaced and very 

 minute granules and microscopic ciliary spinelets. The spinelets on the lower rows of 

 plates are comparatively long and well developed. The character of the alternate pro- 

 minent adambulacral plates is already discernible, although not more than one or two 

 spinelets are present in the armature of each. 



The madreporiform body is outside and external to the interradial or basal plate, and 

 almost in the ravine of the interbrachial arc. The anal aperture is excentric, and 



1 I have given a drawing of this young example in my account of the Asteroidea dredged during the cruise 

 of H.M.S. "Triton" (Trans. Roy. Soc. Edin., 1883, vol. xxxii. pi. xxvi. figs. 9-11). 



