54 
ANIMAL LIFE ON THE 
from the egg, comes forth the pretty round shell we see it to 
be in the adult state. Unlike most other crustaceans that 
cast their shells at various stages of growth, the urchin 
retains its original shell, which grows and expands with the 
body of the creature while life remains. It is not very 
difficult to understand how the whelk enlarges its abode by 
depositing layer upon layer of secretions upon the outer and 
inner lip, like a mason raising and expanding a spiral 
structure, but how different, how wise, nay, how marvellous, 
is the plan of expanding this all but perfect ball. 
Looking at the inner surface of the large spine bands, a 
great many polygonal lines are seen running from top to 
bottom, as shown below. 
SECTION OF urchin's SHELL PLATES. 
With the aid of a good lens, these lines are found to be 
composed of a fibrous tissue dipping from the outer surface, 
where it unites in one continuous web, enveloping the whole 
ball. Thus we see, by the means of this dipping skin, each 
surrounded space of shell is created into a distinct plate 
separate from each other, which in two rows run in sizes 
according to their positions on the curve, making 48 plates, 
and giving in the five larger bands the number of 240. 
Turning to the smaller bands, we can easily discern with the 
naked eye that they are intersected by nearly twice the 
number of lines the former are, being actually divided into 
126 plates, giving in the smaller bands the number of 600, 
making in the entire ball no less than 840 plates. To quote 
Philip Henry Gosse in his Year at the Shore," the theory 
