546 THE FIDDLE HEART-URCHIN. 



are especially valuable just before they deposit their eggs, the roe, as the aggregate egg masses 

 are termed, being large and in as much repute as the "soft roe" of the herring. 



These Sea-urchins are fond of burrowing into the sand, an operation which is conducted 

 mostly by help of the movable spines. They will sink themselves entirely out of sight, but 

 not without leaving a slight funnel-shaped depression in the sand, which is sufficient to guide 

 a practised eye to their hiding-place. 



The Sea-urchins are represented in New England by one species. It is found ensconced 

 in pools among the rocks at low tide, being unaffected by the loss of water during the low tide 

 which leaves them bare. 



The Common Sea-urchin on our coast, bearing the heavy title of Strongylocentrotus 

 drobachiensis (see figure) is the only one quite familiar to Northern waters. It is exceedingly 

 abundant in the tide pools and in the rocky cliffs, and is common in Alaska. A larger species 

 is found in California. The Echinoderms are not very largely represented on the North 

 American coast. In the warmer waters of the Florida Reef they are abundant. 



The accompanying figure (No. 2) is about the average size of our species. The smaller 

 figure (No. 1) represents the young as seen from the side of the mouth. 



The genus to which another species, the PIPER-URCHIN, belongs, can always be recognized 

 by the enormous comparative size of the tubercles sustaining the spines and the parallel rows 

 of ambulacra. 



The members of the genus Cidaris are mostly found in the hotter parts of the world, and 

 are plentiful in the Indian Seaa. The spines of several of the species have been made service- 

 able in the cause of education, being found to make excellent slate-pencils after being calcined. 

 The missionaries have the credit of making this useful discovery. 



The food of the Echini in general seems to consist of various substances, both of an animal 

 and vegetable nature. Fragments of different sea-weeds have been found in the digestive 

 cavity, as also certain portions of shells, which seem to prove that the Echinus had fed upon 

 the mollusks, and broken their shells in pieces with its powerful jaws. The precise mode 

 of feeding is not exactly ascertained ; but it seems likely that the 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 mouth, which is placed in the centre of the 

 oi>en disc. Both univalve and bivalve mollusks appear to be eaten by the Echinus. 



The creature which is represented in the accompanying illustration is appropriately named 

 COMMOK HKART-URCHIIT, from its peculiar shape, and bears 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. 



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 ambu 

 lacra 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 the European 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. 



Another of these remarkable creatures, where the shell is formed into two points, is the 

 FIDDLE HEARTHTRCHIN, so called from the fiddle-shaped mark upon the shell. 



In some of the hotter parts of the world, such as the Indian seas, several species of Echinus 

 are armed with sharp and slender spines, which are apt to pierce the bare foot of a bather, and 

 to cause painful, and even dangerous wounds. Most of these Echini live in the crevices of 

 rocks, but sometimes crawl over the sand, and inflict much suffering upon those who unwit- 



