STRUCTURE OF THE RAYS. 5 



recurved spines, and the entire upper surface of the ray is crowded with stellate paxillae, 

 (fig. 3 B.) 



Fig. 4. 



Portion of a ray of So/aster papposa, Linn. 

 B, the upper surface. 



In the Solasters, the disc is large ; the rays, from twelve to fifteen in number, are 

 short, about half the length of the diameter of the body. In the common Sun-star, 

 Solaster papposa, Lin., one of the most 

 common and handsome of British star- 

 fishes, the structure is well exposed ; fig. 

 4 B exhibits the upper surface, and fig. 

 4 A the under surface of this species.' 

 The disc is large and flat, one half the 

 diameter of the body ; the rays, in 

 general twelve in number, are round 

 and short, their length being about 

 one half the diameter of the disc. 



The entire upper surface is covered with tubercles, those on the disc are scattered 

 over its surface, whilst they are arranged in regular rows on the rays. Each of 

 these tubercles bears a bundle of long, spiculiforra, striated spines, frg. 4 B having from 

 eighteen to twenty grouped in each fasciculus. In the rays there are in general five or 

 six rows of spiniferous tubercles, those on the borders being the largest. The integument 

 between the tubercles is naked, and perforated with many tentacule-pores (fig. 4 B) ; there 

 are no pedicellarige, and the vent is central ; the madreporiform plate is excentral, and 

 its surface is covered with fine radiating lamellae. The under surface of the rays (fig, 4 A) 

 are narrowly lanceolate, the avenues have two rows of suckers, and the ambulacral plates 

 support longitudinal bundles of spines, four or five in each fasciculus. External to these 

 are regular, transverse rows of spines, supported on transverse ridges, eight or ten in 

 each row. " The third series forms a bordering to the arms, and consists of sets of from 

 eighteen to twenty long, fasciculated spines, placed on broad, compressed, articulated 

 bases. The mouth is protected by a beautiful and peculiar mechanism. The angles 

 formed by the joined origins of the rays each bear an ovate sub-triangular plate, grooved 

 down the centre, and carrying two semicircles of long tapering spines, which project 

 in a comb-like manner over the mouth. "^ ■ 



In the structure of the rays, the genus Pteraster resembles some remarkable fossil species. 

 The tegumentary membrane on the convex upper surface is furnished with rows of short 

 spines (fig. 5 B) ; the under surface has a biserial arrangement of tubular suckers in the ambu- 

 lacra, and their margins are provided with numerous transverse fasciculi of spines, five or six 

 in number, the spines of each fasciculus are connected together by a thin membrane, and 



' Forbes, ' British Star-fishes,' p. 113. 



