STAR- PISHES. 
95 
By this beautiful animal we enter into the Echinoder- 
mata, a class of beings much more highly organised than 
any which we have yet considered. Their most promi- 
nent characteristic is, that their softer parts are enclosed 
in what may bo called an external skeleton, a case of cal- 
careous substance, sometimes leathery in texture by the 
predominance of animal matter in the combination, but 
more frequently resembling in its hardness, rigidity, and 
brittleness, the texture of shell or stone. 
If this skeleton, however, wcro made in one unbroken 
piece, it is manifest that there would be no possibility of 
the growth of the animal. As the soft glandular parts 
are all within the shell, every particle of calcareous mat- 
ter deposited would, by being added to the interior sur- 
face, diminish the capacity of the box, and leave less room 
for the vital organs. This emergency is met by a most 
admirable contrivance. If wo take a common sea-urchin 
(Echinus) into our hands, and rub off a few of the spines 
which cluster over its surface, we shall see that its solid 
exterior is a box mado up of a vast multitude of tiny pieces 
of regular shape, fittingtogether at their edges, and soldered, 
as it were, into one, with the most exquisite precision. 
Yet, close as these pieces appear to be to each other, 
and firm as is their adhesion, reason assures us that there 
exists between them a living vascular tissue, of excessive 
tenuity indeed, yet capable of secreting and of depositing 
the materials of growth, in the form of calcareous parti- 
cles, continually added to the edges of the polyhedral 
plates, thus enlarging the capacity of the whole box by the 
slow, even, and imperceptible growth of the thousands of 
constituent pieces. 
