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 :— 
‘*T had not swum very far from the 
beach before I found myself sur- 
rounded by some fifty or sixty human | youne anp apuur SEA-URCHIN.—Strongylocentrotus drobachtensts. 
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 manceuvre, 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 
Vou. III.— 69. 

