SHELL. 



567 



dominate, and the texture may have either a 

 remarkable lightness and porosity, or a con- 

 siderable degree of compactness and brittle- 

 ness. We may take the plates making up 

 the shell of the Echinus as presenting a 

 typical form of this structure ; from which 

 the transition is easy towards either the more 

 solid or the more open character which it 

 elsewhere presents. When we obtain a very 

 thin slice of one of these plates, taken parallel 

 to the surface of the shell, we find that it 

 is composed of a lamina, apparently in itself 

 destitute of structure, perforated with con- 

 siderable regularity by apertures of a circular 

 or oval form. The diameter of these aper- 

 tures (fig. 423.) varies to a certain extent in 



Fig. 423. 



Thin Lamina of shell of Echinus, showing its areolar 

 structure: aa, portions of subjacent layer; bb, 

 fractured bases of columns connecting the super- 

 posed laminae. Magnified 164 diameters. 



different parts of the same shell, the reticula- 

 tion being much coarser in the inner than in 

 the outer layers : from numerous measure- 

 ments, the extremes may be stated at about 

 l-450th and l-2500th of an inch. The en- 

 tire thickness of the shell is made up of an 

 immense number of such plates, which lie 

 parallel to each other, but not in contact ; 

 for they are separated from each other by 

 little pillars, which rise up vertically from 

 each plate to support the next, and which 

 thus connect the different plates whilst hold- 

 ing them apart. The broken bases or ends of 

 these minute pillars are commonly to be seen 

 upon the surfaces of the perforated plates, at 

 the spots intermediate between three or four 

 of the apertures (fg. 423. b, /;). The suc- 

 cessive plates are always so disposed, that the 

 centres of the perforations of one shall corre- 

 spond with the intermediate solid structure 

 of the next (fig. 423. a, a) ; and their trans- 

 parency is such, that, when we have reduced 

 a section to such a degree of thinness as 

 to contain a small number of the reticulated 

 layers, it is easy, by a proper adjustment 

 of the focus of the microscope, to bring 

 either one of them into distinct view. In 

 whatever direction we slice the shell of the 



Echinus, we always meet with a sort of 

 reticulated structure ; for if our section be 

 parallel to the surface of the plates, it brings 

 into view one or more of the perforated 

 laminae just described ; whilst, if it be per- 

 pendicular to the surface, it passes vertically 

 through a series of these laminae, and in the 

 direction of the pillars that connect them, 

 which thus constitute an areolar structure of 

 a tolerably regular form. The testa is thus 

 of an extremely porous character, the areolae 

 having the freest communication with each 

 other. Even in the living state, however, 

 the areolae appear to be empty, the ingress 

 of the fluid with which the surface of the 

 shell is in contact being prevented by the 

 delicate membrane that covers it. At the 

 same time, it possesses a remarkable degree of 

 strength, in proportion to the amount of solid 

 matter employed in its construction ; for 

 every part at the same time supports, and 

 is supported, by the surrounding fabric. 



The skeleton of the Echinodermata con- 

 tains very little organic matter. When it is 

 submitted to the action of dilute acid, so that 

 the calcareous matter is removed, the re- 

 siduum is very small in amount ; indeed, 

 unless the acid be so weak as only just to 

 dissolve the carbonate of lime, the organic 

 matter also will be dissolved, and no animal 

 basis will be apparent. When, however, 

 it is obtained in a state fit for examination, 

 it is found to possess the reticular structure 

 of the calcareous shell ; the meshes or areolae 

 being bounded by a substance in which a 

 fibrous appearance, intermingled with granules, 

 may be discerned under a sufficiently high 

 magnifying power, as was first pointed out by 

 Professor Valentin. This tissue bears a close 

 resemblance to the areolar tissue of higher 

 animals ; and the shell may probably be con- 

 sidered as formed, not by the consolidation 

 of the cells of the epidermis, as in the Mol- 

 lusca, but by the calcification of the fibro- 

 areolar tissue of the true skin. This calcifi- 

 cation of areolar or simply fibrous tissue, 

 by the deposit of mineral substance, not 

 in the meshes of areolae, but in intimate 

 union with the organic basis, is a condition 

 of much interest to the physiologist ; for it 

 presents us with an example, 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, viz. the 

 progressive calcification of the fibrous tissue 

 of the periosteum.* 



Not only the entire shell, but the frame- 

 work by which the teeth of the Echinus are 

 enclosed and supported, is composed of a 

 calcareous reticulation similar to that now 

 described ; nor is it confined to these solid 

 structures. It has been pointed out by Pro- 

 fessor Valentin, that the buccal membrane 

 contains isolated patches of extreme de- 

 licacy ; and the same eminent observer has 

 detected a most beautiful example of this 



* See Dr. Sharpey's Introduction to the Fifth 

 Edition of Dr. Quain's Anatomy, p. 148, et seq. 



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