STAR- FISHES. 



95 



By this beautiful animal we enter into the Eclunoder- 

 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 be 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, were 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 we 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 made up of a vast multitude of tiny pieces 

 of regular shape, fitting together 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. 



