ORIGIN AND SIGNIFICANCE OF SPINES 



13 



Attention should be called to the four kinds of spine 

 production in different organisms. (1) In the Radiolaria, 

 Echinoidea, the Giraife, Cattle, and the Rhinoceros, the 

 spines or horns are persistent, and grow by additions to the 

 original structure. The new tissue may be superficial, sub- 

 terficial, interstitial, or formed by synchronous resorption 

 and growth. (2) In the Crustacea and Articulata generally, 

 and in the Deer, Elk, etc., the spines are moulted, or shed, 

 periodically. In their various stages, these types (1 and 2) 



Figure 15. — Example of spine growth by simple increscence. Horn (left) 

 and horu-core (riglit) of Ox. (After Owen.) 



FiGDRE 16. — Stages of spine growth by successive replacement. Antler series 

 of Red Deer, at ages of 1, 2, 3, etc., years. (After Owen.) 



Figure 17. — Stages of spine growth by serial repetition. Profile of a series 

 of spines on one of the primary radii of Spondijhis imperiuUs. 



Figure 18. — Stages of spine growth by decrescence. Transformation of 

 leaves into spines in Berberis vulgaris. (After Gray.) 



can be studied only by means of separate specimens con- 

 secutive in age, or by observing the metamorphoses in one 

 individual. (3) In the shells of Brachiopoda and MoUusca, 

 the stages of growth of the individual are generally retained 

 throughout life, and the successive development of spines 

 may be studied, therefore, in a single example. (4) Spines 

 produced by suppression, as in the Barberry, express their 

 origin through a series of gradations between separate parts ; 

 while in others suppression is brought about by the loss of 

 structures. 



