REPORT ON THE ASTEROIDEA. 129 
of apit which occupies the median area of the adambulacral plate, and is guarded by a small 
scale-like papilla articulated on the surface of the plate at the side of the pit nearest the 
furrow. ‘The papilla is capable of being shut down, closing the cavity or pit like a lid; 
but as to the function of these organs | am quite ignorant. The cavities are filled with 
a dark-coloured substance which may be either foreign or excreted matter. In one 
species I believe that the pits are aborted on the outer part of the ray, although the 
papilla are present. At present I only know of the existence of segmental pits and 
papillee in two species of Porcellanaster, both from great depths. ‘There are two pits 
and their accompanying papillae on each mouth-plate, and these afford a conspicuous 
indication that each mouth-plate consists of two adambulacral plates fused together 
(Pl. XXVII. figs. 2 and 6). 
Subfamily PorcELLANASTERIN#, Sladen, 1883. 
Genus Porcellanaster, Wyville Thomson. 
Porcellanaster, Wyville Thomson, 1877, Voy. of Challenger, Atlantic, vol, i. p. 378. 
Rays five, comparatively short, upturned at the extremities, and frequently reverted 
over the abactinal area. Disk more or less inflated. 
Supero-marginal plates not united along the median line of the ray, usually bearing a 
spine, and these form a series on each side of the ray. 
Abactinal area covered with membrane, beset wholly or in limited areas with simple 
spiniferous spicules. A more or less elongate tubular epiproctal prolongation is present in 
the centre of the disk, which may be equal in length to the radius of the disk. 
Actinal interradial areas paved with thin plates, more or less regularly disposed, 
but not imbricating and not arranged in definite columns, covered with delicate mem- 
brane. 
Ambulacral furrows wide and exposed. Armature of the adambulacral plates con- 
sisting of one to three elongate, tapering, and sharply pointed spines, usually radiating 
apart. 
Cribriform organs one to three in number. 
Segmental pits and papillze may be present. 
Remarks.—Under favourable conditions, I have detected an extremely minute pore at 
the extremity of the.tubular epiproctal prolongation in Porcellanaster. It is, however, 
so very small that I do not think it can act (in the adult at any rate) as an anal 
aperture, although it may probably be an excretory orifice. (Dr G. O. Sars? has 
1 Researches on the Structure and Affinity of the Genus Brisinga, University-Program, Christiania, 1875, 
p. 50. 
(zooL. CHALL. EXP.—PART LI.—1887. ) 17 
