ORIGIN AND SIGNIFICANCE OF SPINES 83 



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 simi- 

 lar 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 

 therefore the mechanical tissues seldom become hardened. 



Parasitic animals, especially among the Crustacea and 

 insects, 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 Sarcoptes ScahieU 

 and in the female of the parasitic copepod Lernceascus nema- 

 toxys (figure 66). 



Among many aquatic Crustacea and limuloids, the special- 

 ization and segregation of the ambulatory and swimming 

 appendages toward the head or anterior regions of the body 

 have produced a corresponding suppression of appendages on 

 or near the extremity of the abdomen. This statement of 

 fact is the basis of the principle of cephalization of Dana,^^ 

 who applies it especially to the Crustacea, as follows: "There 

 is in general, with the rising grade, an abbreviation relatively 

 of the abdomen, an abbreviation also of the cephalothorax and 

 of the antenntc and other cephalic organs, and a compacting of 

 the structure before and behind; a change in the abdomen 

 from an organ of great size and power and chief reliance in 

 locomotion, to one of diminutive size and no locomotive 

 power." Audouin's law that among the Articulata one part 

 is developed at the expense of another may be also noticed 

 here as affording a further explanation of the suppression of 

 the posterior appendages correlative with the greater develop- 

 ment of the parts anterior to them. In a Crustacean using 

 its tail for propulsion, as the Lobster (^Homarus), the telson 

 is broad and flat, and the adjacent segment has a similar 



