STRUCTURE OF THE SKELETON AND GROWTH. 51 



ambulacral plates in an area, a rather rare variation in this species. This same variation of 

 greater or less degree of radial differential development is shown in many species in this 

 paper. The radial . variation in a specimen may be as great as the variation of different 

 specimens within the limits of a single species. 



Variation is marked as regards the number of ocular plates that meet the periproct in 

 Recent and some Palaeozoic Echini, as fully discussed later. Variation occurs all through 

 Echini in all parts of the organism, and numerous cases are considered in the following pages. 

 All the evidence goes to show that aberrant variation is relatively rare, and in almost all cases 

 variation in Echini is either arrested, progressive, or regressive on the direct line of differential 

 development of the given character in the ontogeny of the individual and the adults of asso- 

 ciated species and genera. A variant in a species can, therefore, usually be compared either 

 as a direct connection, or, if not that, as a parallelism with the typical condition in some other 

 species of the group. 



It seems that Echini are a particularly good group in which to study questions of variation 

 because here variations can usually be expressed in very definite terms of numerical or other 

 equally positive characters. 



Structure of the Skeleton and Growth. 



The skeleton of a sea-urchin is made up of plates, spines, jaws, and other parts which are 

 situated more or less deeply, and the skeleton is covered by living ciliated epithelial tissue. 

 The plates and spines, while apparently external, are in reality included within living tissue. 

 An exception to this covering of living tissue occurs in the primary spines of Cidaris, where, 

 as Lang shows (1896, p. 389), a cortical layer is formed, and the integument dies away from the 

 area covered by that tissue and persists only around the base of the spine. This may he com- 

 pared with the velvet on deer's antlers, which, after the completion of growth, dies away, leav- 

 ing the antler bare. The secondary spines of Cidaris, however, have no cortical layer, and are 

 always covered by a ciliated integument, like a permanent velvet in antlers, and which is seen 

 in the horns of the giraffe and okapi. This condition of the skeleton of Echini, of being covered 

 by living tissue, is of fundamental importance to a proper understanding of the structure. 

 As the parts are internal, they are capable of being added to or resorbed at any part through- 

 out the life of the individual. This is true of all parts except the distal points of the teeth, 

 which are truly external. The individual plate of the test grows by a constant addition to the 

 exterior and resorption of the interior, which latter is composed of open lattice-like or trabecu- 

 lar tissue. The growth of a sea-urchin plate may be aptly compared to the growth of the head 

 of a femur or similar bone with its hard dense exterior and open trabecular internal structure. 

 Sectioning a sea-urchin plate, we find no trace of its earlier shape or character within, any 

 more than we find the traces of a young femur within in sectioning the femur of an adult dog. 



