EEPOKT ON THE ASTEEOIDEA. 129 



of a pit 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 I 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 

 papillas are present. At present I only know of the existence of segmental pits and 

 papillae 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 

 (PI. XXVII. figs. 2 and 6). 



Subfamily Porcellanasterin^e, Sladen, 1883. 

 Genus Porcellanaster, Wyville Thomson. 



Porcellanaster, Wyville Thomson, 1877, Voy. of Challenger, Atlantic, vol. i. p. 378. 



Pays 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 papilke 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. 0. Sars * has 



1 Kesearches on the Structure and Affinity of the Genus Brisinga, University-Program, Christiania, 1875, 

 p. 50. 



(ZOOL. CHALL. EXP. PART LI. 1887.) 17 



