ORIGIN AND SIGNIFICANCE OF SPINES 9 



Definition of Terms. 



In the beginning it is well to understand the meaning and 

 extent of the terms included under the comprehensive word 

 spine. In a general sense, spine is here used to cover any 

 stiff, sharp-pointed process. A prickle is restricted in use 

 to the small, sharp-pointed, conical projections which are 

 purely cuticular; as in the Rose and Blackberry. A thorn 

 is a sharp process on plants, usually representing a branch 

 or stem. A horn is an excrescence on the head of cer- 

 tain animals, and is properly hollow. An antler is a solid 

 bony process, usually 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 

 (Phrynosoma); the pointed spiniform projections on the shells 

 of Mollusca; the spinous prominences on the test of Crus- 

 tacea and insects ; the fin spines as well as those on the oper- 

 cula and scales of fishes ; the generally movable processes of 

 Echinodermata ; 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 equiva- 

 lents. They possess analogy of form without necessary 

 homology of structure, and accordingly have no common 

 phylogenetic connection. Therefore, if the relationships be- 

 tween 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 development, can be ascertained, their relative 



