GENERAL STRUCTURE OF STELLERIDA. 455 



Echinida, we find a stomach only, with a single aperture, like 

 that of the Actinia ( 1057). This stomach has a distinctly radi- 



FlG. GOG. ASTERJAS, OR STAR-FISH. 



ated form. In the Asterias it occupies the centre of the body, 

 and prolongations of it extend into the rays. The side of the 

 body on which the mouth is situated, is always termed the lower 

 surface ; as the Star-fish and its allies generally live in this appa- 

 rently-inverted position. The body and rays of the Stellerida 

 are composed, like the spheroidal shell of the Echinida, of calca- 

 reous plates joined together. The upper surface is usually 

 covered with short spines, moving upon small tubercles, and 

 connected by the general integument, as in the Echinus. It is 

 on the lower surface only, and in the central line of each ray, 

 that we find the ambulacral plates. The tubular feet are very 

 numerous, though short, in the Star -fish; but much fewer in 

 some other forms belonging to the Order. They are connected, 

 as in the Echinus, with a tubular apparatus, which enables the 

 animal to project them either together or separately ; and this 



