464 SPATANGUS. GENERAL STRUCTURE OF STELLER1DA. 



shores, the radiated form is considerably departed from. The 

 shell, instead of being round, is oval ; and it is often much pro- 

 longed in one direction, so that the radial disposition of its parts 

 gives place to a bi-lateral symmetry. We here find neither the 

 mouth nor the vent occupying a central position. Of the habits 

 of the Spatangi little is known. They are almost always found 

 buried in the sand ; and their alimentary canal is filled with the 

 same substance, in which they seem to find enough nutriment, 

 composed probably of the minute animals mingled with it. As 

 they are entirely destitute of teeth, and seem unable to bring 

 their suckers into proximity with the mouth, they must derive 

 their nourishment from the chance-supplies which the sub- 

 stances in contact with their mouths may furnish. But their 

 whole organization is adapted to this mode of existence ; as much 

 as the more complex structure of the Cidarites is to the life 

 which they are intended to lead. Yet it is difficult to imagine 

 how, with so little power either of locomotion or prehension, 

 they can obtain the necessary amount of aliment. 



ORDER II. STELLERIDA. 



1103. In this Order, of which the common Asterias, or Star- 

 fish, may he taken as the type, we approach in a certain degree 

 to the inferior forms of Radiata. Instead of the complete diges- 

 tive apparatus, with two orifices, which we have seen in the 

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

 like that of the Actinia ( 11 21). This stomach has a distinctly 

 radiated 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 



