SPINES ; PEDICELLAKI^: 889 



and thus it would appear that the entire spine must be formed at 

 once, since no addition could be made either to its length or to its 

 diameter, save on the outside of the sheath, where it is never to be 

 found. The sheath itself often rises up in prominent points or 

 ridges on the surface of these spines ; but, as is shown in fig. 676, 

 the reticular portion may have a share in the formation of the rings. 

 This view of the mode of formation of the Cidarid spine is con- 

 tested by Professor Jeffrey Bell, who has brought forward l evidence 

 to show that if two spines of different sizes be taken from two 

 examples of Cidaris metularia, also differing in size, the quantity of 

 solid calcareous sheath seen in transverse section is proportionately 

 less in the larger than in the smaller spine ; from this he concludes 

 that the growth is due to the internal reticulated portion rather 

 than to the outer crust. The slender, almost filamentary spines 



FIG. 677. Spine of Sputangus. 



of Spatangus (fig. 677) and the innumerable minute hair-like pro- 

 cesses attached to the shell of Clypeaster are composed of the like 

 regularly reticulated substance ; 2 and these are very beautiful objects 

 for the lower powers of the microscope, when laid upon a black 

 ground and examined by reflected light without any further prepara- 

 tion. It is interesting also to find that the same structure presents 

 itself in the curious Pedicellarice (forceps-like bodies often mounted on 

 long stalks), which are found on the surface of many Echinida and 

 Asterida, and the nature of which was formerly a source of much 

 perplexity to naturalists, some having maintained that they were 

 parasites, whilst others considered them as proper appendages of the 

 Echinus itself. The complete conformity which exists between the 

 structure of their skeleton and that of the animal to which they are 

 attached removes all doubt of their being truly appendages to it, as 

 observation of their actions in the living state would indicate. 3 



1 Journ. Roy. Microsc. Soc. 1884, p. 845. 



2 A number of rare spines are described and figured by Prof. H. W. Mackintosh 

 in vols. xxvi. (p. 475) and xxviii. (pp. 241 and 259) of the Trans. Boy. Irish Academy. 



5 Prof. Alex. Agassiz has shown the relations of the Pedicellariae to the spines. 

 Much information regarding the various forms of these curious bodies will be found 

 in Professor Perrier's memoir in the Ann. Sc. Nat, (5), vols. xii. and xiii. ; Mr Sladen's 



