ORIGIN AND SIGNIFICANCE OF SPINES 17 



many of the forms are repeated again and again, not only 

 in various species of this group, but elsewhere both in the 

 animal and vegetable kingdoms. 



Whenever the development of a compound spine can be 

 studied, it shows a gradual progress from the simple to the 

 complex (figures 21-23). The antlers of the Red Deer 

 (^Cervus elaphus) furnish a familiar example. Fawns of the 

 first year have antlers with only a single prong, a short front 

 tine being added the second year; then "year by year as 

 they are renewed they acquire a greater and still greater 

 number of tines and branches, till they finally attain the 

 complete stage, when their owner is termed a ' royal hart ' " ** 

 (figure 16). Although somewhat conventionalized, the pri- 

 mary series of spines on the Sjjondylus shown in figure 20 

 exhibits the passage from simple to compound forms. An 

 inspection of many species of Miirex will show the stages in 

 series presenting a greater complexity. 



After spine development has reached its maximum growth 

 and differentiation, evidence of old age may be exhibited in 

 two ways: (a) The spines may be reduced by resorption, 

 decay, or abrasion, and finally become obsolescent; or what 

 is of greater import (1)\ they may gradually cease to be 

 developed, as is especially shown in organisms in which spine 

 growth is by serial repetition. Thus, in Spondylus calcifer, 

 a young individual measuring about two inches across has 

 marginal spines fully an inch in length. Even longer spines 

 are found when the shell reaches a width of four inches. On 

 attaining a maximum diameter of about six inches, spine 

 growth gradually ceases, and the margin of the valves is 

 entire and nearly smooth. At this stage shell secretion is 

 confined to excessive thickening of the valves. These senile 

 stages of spine growth will receive further consideration 

 under the discussion of ontogeny and phylogeny of spinous 

 species. 



Application of Law of Morphogenesis. — The manner in 

 which spines arise from plane surfaces, or from the growth 



2 



