ECHINODERMATA. 165 



the round ligament found in the hip-joint, and obviously a provi- 

 sion for the prevention of dislocation. 



The whole joint is moreover enclosed in a muscular capsule, 

 composed of longitudinal fibres (b, b) arising from the circum- 

 ference of each tubercle, and inserted all around the root of the 

 spine : these fibres therefore, which must in fact be regarded as 

 merely derived from the general irritable skin that clothes the shell 

 externally, are the agents which, acting immediately upon the 

 spine, produce all the movements of which it is capable. 



(206.) The next thing to be accounted for in the history of 

 these elaborately constructed animals is the growth of the spines 

 themselves, which, as we have already seen, are completely 

 detached from the rest of the shell, to which they are only 

 secured by the central ligament, and by the muscular capsule 

 enclosing their base. To account, therefore, for the production 

 of organs so completely insulated as the spines appear to be, 

 especially when we consider that there is no vascular communi- 

 cation between them and the body of the Echinus, would appear 

 to be a matter of some difficulty; and in fact r had we not already 

 seen in the polyps the amazing facility with which calcareous 

 matter was secreted by the living textures of those animals, ifc 

 would be almost impossible to conceive by what process their 

 growth was effected. On examining one of these appendages, taken 

 from a species in which they are largely developed, when fresh, 

 before its parts have become dry, every portion of its surface is 

 seen to be invested with a thin coat of soft membrane, derived 

 from that which covers and secretes the whole shell, of which 

 indeed the muscular capsule enclosing its articulation with the 

 tubercle is only a thickened portion. 



The living covering of the spine therefore, like the crust which 

 invests the cortical polyps, is the secreting organ provided for its 

 growth, depositing the earthy particles separated from the waters of 

 the ocean, layer after layer, upon its outer surface, so as to form a 

 succession of concentric laminae, of which the outer one is always 

 the^last formed. The calcareous matter thus deposited has more 

 or less completely a crystallized appearance ; and on a transverse 

 section of the organ being made, and the surface polished by 

 grinding, the whole process of its formation is at once rendered 

 evident. Such sections, indeed, form extremely beautiful and 

 interesting subjects for microscopical examination, as nothing can 

 exceed the minute accuracy and mathematical precision with which 



