194 ECHINODERMATA. 



(508.) Moreover, the whole joint is enclosed in a muscular capsule, 

 composed of longitudinal fibres (b b) arising from the circumference 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 on the spine, produce all the move- 

 ments whereof it is capable. 



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

 elaborately-constructed animals is the growth of the spines themselves : 

 these, as we have already seen, are completely detached from the rest of 

 the shell, to which they are secured only 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 communication 

 between them and the body of the Echinus, would appear to be a 

 matter of some difficulty ; and in fact, had we not already seen, in the 

 Polyps, the amazing facility with which calcareous matter was secreted 

 by the living textures of those animals, it would be almost impossible 

 to conceive by what process their growth was effected. On examining 

 one of these appendages, taken from a species wherein 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, whereof 

 indeed the muscular capsule enclosing its articulation with the tubercle 

 is only a thickened portion. 



(510.) The living covering of the spine, therefore, like the crust 

 that 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 succes- 

 sion of concentric laminae, of which the outer one is always the last 

 formed. The calcareous matter thus deposited has, more or less com- 

 pletely, 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 micro- 

 scopical examination, as nothing can exceed the minute accuracy and 

 mathematical precision with which each particle of every layer com- 

 posing them appears to have been deposited in its proper place : in fact, 

 if the zootomist would fully appreciate the minuter details connected 

 with their organization, it is only by the employment of the microscope 

 that he will arrive at adequate ideas concerning them ; for it is not in 

 the number and variety of the pieces entering into the composition of 

 the skeleton of one of these animals, the extraordinary apparatus of 

 prehensile suckers with which they are furnished, or the singular 

 locomotive spines upon the exterior of the shell, that he will find the 



