480 



OF ECHINODERMATA. 



which is continuous with that covering the surface of the shell, 

 and which serves not merely to hold down the cup upon the 

 tubercle over which it works, but also, by its contractility, to 

 move the spine in any required direction. This membrane is 

 probably continued onwards over the whole surface of the spine, 

 although it cannot be clearly traced to any distance from the 

 base ; and the new formations may be presumed to take place in 

 its substance. Each new formation completely ensheaths the 

 old; not merely surrounding the part previously formed, but 

 also projecting considerably beyond it ; and thus it happens that 

 the number of layers shown in a transverse section, will depend 

 in part upon the place of the section. For if it cross near the 

 base, it will traverse every one of the successive layers from the 

 very commencement ; whilst, if it cross near the apex, it will 

 traverse only the single layer of the last growth, notwithstanding 

 that, in the club-shaped spines, this terminal portion may be of 

 considerably larger diameter than the basal ; and in any inter- 

 mediate part of the spine, so many layers will be traversed as 

 have been foi'med since the spine iirst attained that length. The 

 basal portion of the spine is enveloped in a reticulation of a very 

 close texture, without concentric layers ; forming the cup or 

 socket which works over the tubercle of the shell. The combi- 

 nation of elegance of pattern Avitli richness of coloring, renders 

 well-prepared specimens of these spines among the most beauti- 

 ful objects that the Microscopist can anywhere meet with. The 

 large spines of the various species of the genus Acrocladia fur- 

 nish sections most remarkable for size and elaborateness as well 

 as for depth of color (in which last point, however, the deep 

 purple spines of Echinus Jividus are pre-eminent) ; but for exqui- 

 site neatness of pattern, there are no spines that can approach 

 those of Eehinometra heteropora and E. lucunter. The spines of 

 Helioeidaris variolaris are also remarkable for their beauty. No 

 succession of concentric layers is seen in the spines of the British 

 Echini, probably because (according to the opinion of the late 

 Sir J. G. Dalyell) these spines are cast off and renewed every 

 year ; each new formation thus going to make an entire spine, 

 instead of making an addition to that previously existing. 

 Most curious indications are sometimes afforded by sections of 

 Echinus-spines, of an extraordinary power of reparation inherent 

 in these bodies. For irregularities are often seen in the trans- 

 verse sections, which can be accounted for in no other way, than 

 by supposing the spines to have received an injury when the 

 irregular part was at the exterior, and to have had its loss of 

 substance supplied by the growth of new tissue, over which the 

 subsequent layers have been formed as usual. And sometimes 

 a peculiar ring may be seen upon the surface of a spine, which 

 indicates the place of a complete fracture, all beyond it iDeing a 

 new growth, whose unconformablenoss to the older or basal 

 portion is clearly shown by a longitudinal section. 



