6 0. E. Beecher — Origin and Significance of Spines. 



deciduous, and generally confined to the male; as in the Deer 

 or Elk. A spur is a term applied to the claw-like process on 

 the legs and wings of some birds, and on the hind legs of 

 Ornithorhynchus and Echidna. 



The word spine, therefore, is most comprehensive, and is 

 here intended to include the modified hairs of the Echidna 

 and Porcupine ; the sharp, prickly scales of the Horned Toad 

 (Phrynosomd) / the pointed spiniform projections on the 

 shells of Mollusca; the spinous prominences on the test of 

 Crustacea and insects ; the fin spines as well as those on the 

 opercula and scales of fishes ; the generally movable processes 

 of Echinoderms ; the projecting rays and processes of Radio- 

 laria, etc., etc. The vertebral column and also the processes 

 from the separate vertebrae are known as spines, but as these 

 are distinctly internal structures, they will not be considered in 

 this connection. 



In nearly all classes of organisms, spines have been devel- 

 oped independently, and simply represent cases of parallel 

 development of similar structures or morphological equivalents. 

 They possess analogy of form without necessary homology of 

 structure, and accordingly have no common phylogenetic con- 

 nection. Therefore, if the relationships between the smooth 

 and spinose forms belonging to any group of animals or plants 

 can be traced, and the simplest and most primitive condition 

 in each case, as well as the highest stage of progressive develop- 

 ment, can be ascertained, their relative significance from an 

 evolutionary standpoint may be confidently determined. 



Growth of a Spine. 



The growth of a spine is either direct and progressive, or 

 indirect and regressive. It is direct when it is developed by 

 the addition of new tissue. In this way, growth is attained in 

 the antlers of a Deer, the horns of a Cow, the ordinary spines 

 of Brachiopods, Mollusca, and Crustacea, and in other similar 

 examples covering the majority of cases. Growth is indirect, 

 however, when the spine represents atrophy or suppression of 

 an organ through the loss of its accessory parts, as in the 

 thorns of the Locust and the Barberry, the spiniform termina- 

 tion of the stems of the Pear, or the spurs on the Python. 



The direct development of a spine is essentially the same 

 process in all cases. At a given point on the surface of an 

 organism, there first appears a slight elevation, which becomes 

 higher and higher, and is usually conical in form. This cone 

 represents the simplest type of spine ; and among animals and 

 plants, most spines conform to this primitive pattern (figures 

 1-5). 



