250 THE MICROSCOPE. 



greater number of spines the sections of the pillars present no struc- 

 ture, in others they exhibit a series of concentric rings of successive 

 growth, which strongly remind us of the medullary rays of plants ; 

 occasionally they are traversed by reticulated structures, as represented 

 in Plate VI. No. 1. When a vertical section of a spine is examined, it 

 will be found to be composed of a series of cones placed one over the 

 other ; the outer margin of each cone being formed by the series of 

 pillars. In the genus Echinus, the number of cones is considerable, 

 while in that of Cidaris there are seldom more than one or two ; so that 

 from such species transverse sections may be made, having no concen- 

 tric rings, and in which only the external row of pillars may be seen. 



" The skeleton of Echino-dermata contains very little organic mat- 

 ter. When it is submitted to the action of very dilute acid, to dis- 

 solve out the calcareous matter, the residuum is very small in amount. 

 When obtained, it is found to possess the reticular structure of the 

 calcareous shell (Plate VI. No. 7); the meshes or areolse being bounded 

 by a substance in which a fibrous appearance, intermingled with gra- 

 nules, may be discerned under a sufficiently high magnifying, power, as 

 was first pointed out by Professor Valentine. This tissue bears a close 

 resemblance to the areolar tissue of higher animals ; and the shell may 

 probably be considered as formed, not by the consolidation of the cells 

 of the epidermis, as in the mollusca, but by the calcification of the fibro- 

 areolar tissue of the true skin. This calcification of areolar or simply 

 fibrous tissue, by the deposit of mineral substance, not in the meshes 

 of areolaB, but in intimate union with the organic basis, is a condition 

 of much interest to the physiologist ; for it presents us with an ex- 

 ample, even in this low grade of the animal kingdom, of a process 

 which seems to have an important share in the formation and growth 

 of bone, namely, in the progressive calcification of the fibrous tissue of 

 the periosteum membrane covering the bone."* 



From their peculiarity of structure, they may be said to be almost 

 imperishable. Their shells exist abundantly in all our chalky cliffs, 

 innumerable specimens of which may be obtained, exhibiting the 

 same wondrous forms and characters as those which now frequent 

 our shores. 



The Crinoidea, or Sea-lilies, so called from the resemblance which 

 many of them present to flowers, were exceedingly abundant in for- 

 mer ages of the world ; and their remains often form the great bulk of 

 large masses of rock. During the whole or a part of their existence, 



* Dr. Carpenter, Cyclopaedia of Anatomy and Physiology. 



