THE COMMON SEA-URCHIN. 



545 



The shell is composed of a vast number of separate pieces, whose junction is evident 

 when the interior of the shell is examined, but is almost entirely hidden by the projec- 

 tions upon the outer surface. These pieces are of a hexagonal or pentagonal shape, 

 with a slight curve, and having mostly two opposite sides much longer than the others. 

 As the animal grows, fresh deposits of chalky matter are made upon the edges of each 

 plate, so that the plates increase 

 regularly in size, still keeping their 

 shape, and in consequence the dimen- 

 sions of the whole shell increase, while 

 the globular shape is preserved. 



If a fresh and perfect specimen 

 be examined, the surface is seen to 

 be covered with short sharp spines 

 set so thickly that the substance of 

 the shell can hardly be seen through 

 them. The structure of these spines 

 is very remarkable, and under the 

 microscope they present some most 

 interesting details. Moreover, each 

 spine is movable at the will of the 

 owner, and works upon a true ball- 

 and-socket joint, the ball being a 

 round globular projection on the sur- 

 face of the shell, and the socket 

 sunk into the base of the spine. 

 When the creature is dead and 

 dried, the membrane which binds to- 

 gether the ball-and-socket joint be- 

 comes very fragile, so that at a slight 

 touch the membrane is broken and 

 the spines fall off. 



Other peculiarities of structure 

 will be noted in connection with the 

 different species. 



The common Sea-urchin is edible, 

 and in some places is extensively 

 consumed, fully earning its title of 

 Sea-egg, by being boiled and eaten 

 in the same manner as the eggs of 

 poultry. 



The fishing for these creatures in 

 the Bay of Naples is graphically and 

 quaintly described by Mr. R. Jones : 

 "I had not swum very far from the 

 beach before I found myself sur- 

 rounded by some fifty or sixty human 

 heads, the bodies belonging to which 



were invisible, and interspersed among these, perhaps, an equal number of pairs of feet 

 sticking out of the water. As I approached the spot, the entire scene became sufficiently 



ludicrous and bewildering Down went a head, up came a pair of heels down went a 



pair of heels, up came a head ; and as something like a hundred people were all diligently 

 practising the same manoeuvre, the strange vicissitude from "heels to head and head to heels, 

 going on simultaneously, was rather a puzzling spectacle." 



After inquiry, it proved that these divers were engaged in fishing for Sea-urchins, which 



VOL. m.-89. 



YOUNG AND ADULT SEA-URCHIN. 8trongytocentrotu 



