0. E. Beecher — Origin and Significance of Spines. 341 



pies of spines produced by decrescence may belong to more 

 than one category. However it does not prevent the accept- 

 ance of any one of the three as primary causes. Thus, it may 

 be urged that disuse has caused the atrophy of leaves into spines 

 among many desert plants, or produced a similar reduction of 

 the limbs in a Python. While this may be true from one 

 point of view, yet the manifest unfavorableness of the eviron- 

 ment in both, seems to be a sufficient reason for making it 

 the primary factor. On the other hand, many parasites show- 

 ing similar atrophies are not dependent upon a large number 

 of active organs for their food and maintenance. After find- 

 ing a host, an abundance of food is at hand, and the environ- 

 ment may be considered a favorable one. All the organs, 

 except those of nutrition and reproduction, then become more 

 or less nseless and dwindle away, leaving vestigial organs, or 

 disappearing altogether. Furthermore, a change of habit, as 

 from climbing to flying, will necessarily cause the atrophy of 

 some of the structures used for climbing, and the hypertrophy 

 of others for flying. 



Most of the examples illustrating the production of a spine 

 through the atrophy of an organ by disuse are to be found in 

 the legs and digits of animals. The process bears consider- 

 able resemblance to the formation of spines on many plants by 

 the suppression of leaves, branches, etc. They will be noticed 

 here, although properly these vestigial structures among ani- 

 mals are more strictly of the nature of claws, or at the most, 

 spurs. 



Many parasitic plants, especially among the Balanophorese, 

 are reduced to a simple stem bearing the inflorescence. The 

 leaves are represented by scales which are often spiniform, 

 though seldom of sufficient stiffness to entitle them to be 

 called spines. In desert plants, many of which have a similar 

 type of growth, the hardening of the mechanical tissues by 

 the effects of drought has converted similar leaf structures 

 into spines, while the parasitic plants are not normally sub- 

 jected to such continuous dryness and extreme heat, and there- 

 fore the mechanical tissues seldom become hardened. 



Parasitic animals, especially among the Crustacea and in- 

 sects, often show a reduction in the number of joints in the 

 legs, and even in the number of limbs themselves. The 

 terminal claws generally persist, and are sometimes longer than 

 the rest of the leg ; as in the Itch mite, Sarcojjtes Scabei, 

 and in the female of the parasitic Copepod Lernwascus nema- 

 toxys (figure 66). 



Among many aquatic Crustacea and Limuloids, the specializa- 

 tion and seggregation of the ambulatory and swimming append- 

 ages towards the head or anterior regions of the body have 



