THE HEART-UECHIN. 



725 



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

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



PIPER-URCHIN. Cidaris papillate 



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 726 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 fossL 

 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 plentiful, 

 and mostly appear to live below the sand. They seem to feed on the animal substances 

 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. 



