454 CLYPEASTER ; SCUTELLA ; SPATANGUS. 



known of these genera, in consequence of its more extensive 

 geographical distribution, and its abundance in the places it 

 frequents. One species, found on the coast of Ireland, has the 

 power of excavating for itself hollows in the limestone rock on 

 which it lies ; but by what means it does so is uncertain. In 

 the Clypeaster and Scutella, the shell is more or less flattened ; 

 and is sometimes divided by vertical partitions, so as in some 

 degree to resemble a Star-fish. At the same time, the vent 

 leaves its central position on the upper surface, and approaches 

 the mouth, which still retains its central position below. In 

 the Spatangus, or Heart- Urchin, common on many of our sandy 

 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 rfeither 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 substances 

 in contact with their mouths may furnish. But their whole 

 organisation 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. 



1012. In this Order, of which the common Asterias, or Star- 

 fish, may be 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 



