THE HEART-URCHIN. 



725 



Echinus can seize its prey with any of its ambulacra, no matter on what portion of the 

 body they may be situated, and pass it from one to the other until it reaches the 



PIPER-URCHIN. Cldarls papillate. 



mouth, which is placed in the centre of the open disk. Both univalve and bivalve 

 molluscs appear to be eaten by the Echinus. 



THE creatures which are represented in the illustration on page 7 26 are appropriately 

 named HEART-URCHINS, from their peculiar shape, and bear an evident resemblance to 

 the heart-cockles already mentioned. Many species of Heart-urchins are found in a 

 fossil state, and are especially common in the chalk formations. On the upper right- 

 hand side of the illustration is seen the handsome PURPLE HEART-URCHIN in a perfect 

 state, with all its array of slender armed spines; while on the other side, is a specimen 

 of the COMMON HEART-URCHIN with its spines removed, showing well its peculiar 

 shape. 



The shell of this genus is slight and delicate, and is composed of very large plates, 

 which, in consequence, are comparatively few in number. There is always a furrow 

 of greater or less depth at the upper end. In the naked specimen the rows of pores 

 through which the ambulacra pass are plainly perceptible, and even in the fossilized 

 specimens, which have been buried in the earth for so many ages, these pores are still 

 visible, and so plainly marked, that the genus and species of the dead shell can be 

 made out with little less ease than if the animal were just taken out of the water. 



The Heart-urchins are found in all parts of the world, and our own seas contain 

 specimens of these curious beings. In the Mediterranean they are extremely plenti- 

 ful, and mostly appear to live below the sand. They seem to feed on the animal sub- 

 stances that are mingled with the sand, for M. de Blainville found, on dissecting many 

 specimens, that their digestive organs were always filled with fine sand. The walls of the 

 digestive cavities are exceedingly delicate, and have been compared to the spider's web. 



In the lower left-hand corner may be seen another form of these remarkable 

 creatures, where the shell is formed into two points. This is the FIDDLE HEART-URCHIN, 

 so called from the fiddle-shaped mark upon the shell. At its right hand is another 

 specimen denuded of its spines, for the purpose of showing the peculiar mark from 

 which the species derives its name. The Common Heart-urchin with all its spines is 

 seen in the right-hand lower corner. All these species are found in the British seas. 



