Marvels of the Universe 



473 



Phoio ?.o] 



[/;. W(V), F.L.S. 



THE HEART-URCHIN. 



This species burrows in fine sand and mud. It is covered with fine bristles (cleaned off the middle specimen*, and has 

 the pores all on the upper side. (Slightly reduced from natural size.) 



that make up the Urchin's box, and this is true ; yet the plates are not in actual contact, for a 

 film of living animal matter separates them. It is by the vitality of this film that the box 

 increases in size, for it is always depositing fresh particles of lime equally on all the edges of all 

 the plates, so that increase takes place simultaneously in all parts. The Urchin multiplies by 

 means of eggs, and the young go through preliminary stages of existence in which they present 

 not the faintest resemblance to their parents. But the adult form in little gradually develops 

 within the infant body, which is ultimately thrown off. 



In the breeding season our Urchin contains five large masses of eggs, and in some localities 

 on our own coasts these make it a desirable article of food. On the shores of the Mediterranean 

 also Urchins are esteemed, and at Naples they are known as friitta di mare, or fruit of the sea. So 

 great is their fecundity that it is calculated that our Common Sea-Urchin produces twenty 

 millions of eggs in a single season. 



There are throughout all seas many variations from this typical form, and a few of them may 



flMto hy] [i-. step, F.L.S. 



THE WHEEL-URCHIN. 



An African species of Cake-Urchin which burrows in the sand. The spines have been cleaned off. One half the 

 margin is indented to form teeth like the cogs of a wheel. (Natural size.) 



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