504 



THE MICROSCOPE. 



from these species transverse sections may be made, having 

 no concentric rings, and in which only the external row of 

 pillars can be seen. 



" The skeleton of the Echinodermata contains very little 

 organic matter. When it is submitted to the action of 

 very dilute acid, to dissolve 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 (Fig. 240, No. 1) ; the meshes or areolae being bounded 



Fig- 240. 



1, Section of spine of Echinus, exhibiting reticulated structure, the calcareous 

 portion haying been dissolved out by acid. 2, Transverse section of shell 

 of the Pinna ingens. 3, Horizontal section of shell of Tercbratulaia rubicunda, 

 showing its radiating perforations. 



by a substance in which a fibrous appearance, intermingled 

 with granules, may be discerned under a sufficiently high 

 magnifying power, as originally pointed out by Professor 

 Valentine. This tissue bears a close resemblance to the 

 areolar tissue of higher animals ; and the shell may pro- 

 bably be considered as formed, not by the consolidation of 

 the cells of the epidermis, as in the 3follusca, but by tlie 

 calcification of the fibro-areolar tissue of the true skin. 



