STAR-FISHES AND SEA-URCHINS. 59 



skin of an orange, leaving them united at the base \ 

 then peel it off, and stretch it out, you have a star with 

 five rays ; draw the rays together and unite them at 

 the top, and you have again the round form of the 



orange. 



The Sea-Urchin has one very peculiar habit. He 

 bores for himself a hole in the rocks, which just fits 

 him, and makes a very snug and comfortable retreat. 

 I have seen a dead Sea-Urchin, about as large round as 

 a five-cent piece, packed away as closely as possible in 

 its hole, that fitted him as neatly as if it had been cut 

 with the nicest instrument. Their mode of making these 

 holes is not known ; and as they are found in all kinds 

 of rocks whether hard or soft, where Sea-Urchins exist, 

 in granite or basalt as well as in limestone or sand- 

 stone, it is difficult to understand how animals not 

 furnished with any sharp and powerful instruments can 

 produce such an effect. There is, however, no doubt 

 that these holes are made by the animals themselves, 

 not only because the Sea-Urchins are found in them, 

 but because they fit their inhabitants so perfectly that 

 no animal not exactly of the same shape and size could 

 have produced them ; and they are of all sizes, from 

 that of the young Sea-Urchin to the full-grown one. 



It has been supposed by some naturalists that they 

 were made by the constant friction of a fringe that is 

 in unceasing motion, called the vibrating cilia ; this 

 fringe, though invisible to the naked eye, covers the 

 spines of the Sea-Urchin, and, by the constant turning 

 of the animal over and over in the same spot, may 

 wear a hole in the rock. It seems difficult to believe 

 that a substance so soft and delicate as the vibrating 



