ORIGIN AND SIGNIFICANCE OF SPINES 19 



nauplius of Crustacea and the protaspis of Trilobita are 

 generally spineless. The young of horned vertebrates are 

 almost universally hornless, the Giraffe being the only mam- 

 mal born with horns. The very young seedlings of plants 

 are likewise sj)ineless. In insects the embryonic stages 

 generally have simple cuticles, but in the larval stages of 

 this class and the Crustacea, a great variety of spines and 

 ornamental characters is developed. Altogether, it may be 

 asserted that spines do not appear during the embryonic 

 stages of animals and plants, and that their initial develop- 

 ment is commonly post-larval. 



Examples illustrating the ontogeny of a spinose form could 

 be multiplied indefinitely, and taken from nearly every class 

 of organisms. In all cases practically the same sequence of 

 events relating to the development of spines would be found. 

 The organism would fii"st be smooth, without sculpture or 

 ornament, like the young of other organisms. At some stage 

 of the ontogeny the beginnings of spines would appear, and 

 develop first into simple, and later, according to the stage of 

 differentiation attained, into compound spines. This pro- 

 gression would finally reach the maximum, spine growth 

 would cease, and the surface of the organism would inversely 

 revert to an early and more primitive type without spines. 

 Normally these changes would represent the infantile, ado- 

 lescent, mature, and early and late senile periods of the life 

 of the organism. In some cases, however, the stages of spine 

 growth, or acanthogeny, do not agree with the ontogeny of 

 the entire individual in respect to time, and here acceleration 

 and the phylogeny of the species will be found to offer the 

 proper explanation of the divergence. 



As simple examples of the ontogeny of spiniferous species, 

 the jMollusca afford especial advantages, owing to the fact 

 already noticed, that the stages of develoj)ment are commonly 

 preserved in a single individual. In figure 24 the larval 

 shell, or prodissoconch, of Pelecypoda, or bivalve shells, is 

 represented, and shows the usual type throughout a large 

 portion of the class. The succeeding shell growth of the 



