ASTEROIDEA 221 



slender acicular spine and two shorter ones at mouth of furrow. One or both of these 

 lateral spinelets may be suppressed by fusion of plates. Actually the actinostomial 

 margin of the combined mouth-plates of any oral angle carries 2 or 3 unequal spines, 

 except when one " corner " has failed to fuse. On that particular plate there are 3 spines, 

 the apical and 2 laterals. The companion plate of that angle (if fused with its vis-d-vis of 

 the next angle) has only the apical spine surviving. The very constricted outer part of 

 each oral pair carries 1-3 prominent slender acicular suborals irregularly placed. 



The circle of mostly fused mouth-plates overhangs the outer part of the peristome so 

 that the circumoral nerve is at the rear, or bottom, of a rather deep groove above the 

 projecting oral angles. The cheval-de-frise of spines and large pedunculate straight 

 pedicellariae evidently provide an efficient protection to the nerve. 



Behind the oral pair of plates there is an extensive adoral carina composed of 6-8 pairs 

 of juxtaposed adambulacral plates of adjacent rays. Each of these plates carries i spine, 

 the inner of the pair characteristic of adambulacrals of the free part of the ray. 



In spite of a certain amount of variation in detail, the two species of Labidioster are 

 easy to distinguish, the general habit of medium-sized specimens being well shown by 

 Sladen's figures which, however, do not do justice to the disparity in size of the crossed 

 pedicellariae. In radiosus these pedicellariae are not only smaller, but differ slightly in 

 detail. The skeletal arches, in a rudimentary form, persist to the end of ray. The disk is 

 smaller, while a scattering of primary spines is conspicuously larger than the second- 

 aries ; in amiidatiis there is no such disparity, the spines being uniformly small and more 

 numerous. In radiosus straight pedicellariae are not numerous, but this feature may 

 prove to be variable. 



In young specimens of amiidatus (e.g. R 50 mm., r 12 mm.) the transverse skeletal 

 arches persist to the end of the ray as in adult radiosus. Verrill made a "book-genus", 

 Labidiastrella for L. amiidatus,^ based upon the absence of the dorsal skeleton distally. 

 The difference between radiosus and ammlatus in this respect is one of degree, not kind. 

 The abactinal skeleton of radiosus is incomplete distally, but the degeneration has not 

 been carried so far as in ammlatus. Compensating for the loss of abactinal plates, 

 anmdatus has developed a remarkable equipment of crossed pedicellariae of very large size . 

 As in Pycnopodia, the loss of the abactinal skeleton undoubtedly confers a high degree 

 of flexibility to the rays. Active prey can be detained long enough by the large crossed 

 pedicellariae for the tube-feet of adjacent rays to be brought into action. Rapid torsion 

 of rays is of distinct advantage. In the stomach of a rather small example, with disk 

 diameter of 30 mm., are fragments of a Euphausia (probably superba) about 40 mm. 

 long (St. 190). A specimen from St. 371 and one from St. 1955 have captured isopods; 

 one from St. 148 contains a small ophiuran. In general the mouth is widely open and 

 sometimes the stomach is partly everted. 



Koehler's records of L. radiosus from the Antarctic Archipelago refer to L. ammlatus. 

 His figure (1912, pi. i, fig. i) of a specimen from 64° 48' S, 65° 51' W is clearly referable 

 to atmidatus. 



1 Verrill, 1914, p. 352. 



