based on the Princple of Cephalization. 



85 



* h. An abnormal enlargement of the general structure. 



The elongation or enlargement which takes place with decline 

 of grade is mQ.m\y posterior^ it being small anteriorly, and some- 

 times none at all. In passing from the Brachyural to the Mac- 

 rural type of Crustaceans, the change anteriorly is principally in 

 an increased laxness and lengthening of the parts, with little in- 

 crease in the dimensions of the body anterior to the mouth ; while 

 the abdomen (or posterior extremity) is enlarged 10 to 50 times 

 beyond the bulk it has in the Crab. Descending from a snail to 

 an oyster, there is diminution anteriorly and great enlargement 

 posteriorly, and the animal is little more than a visceral sac. 



In less marked cases of the amplificutive method, there is only 

 an attenuation or lengthening of the body and limbs, as in many 

 Neuropters, Or diopters, Homopters, wading Birds, &c. The 

 Lepidopters, also, in their very great expanse of wing, exemplify 

 this method. In species that are attached, as the Cirripeds, the 

 young are usually free ; and it is only when they begin to out- 

 grow, amplificately, the minute life-system (Entomostracan in 

 the Cirripeds) that they become tixed. As attached animals, 

 they often attain great size. 



Viewed on the ascending grade, this method is the concentra- 

 tive; and it is exhibited in the increased abbreviation and conden- 

 sation of the anterior and posterior members and segments, or of 

 the whole structure. 



6. Multiplicative. — Exhibited in an abnormal multiplication of 

 segments or members ; as in Myriapods, Worms, Phyllopods, 

 Trilobites, &c. There may be — 



a. Simple Multiplicative ; as in the superior Myriapods, the 

 Chilopods, in which the body-segments, thus multiplied, have each 

 its single or normal pair of members. 



h. Compound Multiplicative ; as in the Myriapods, of the lulus 

 division, or Diplopods (Chilognaths), in which there is a duplica- 

 tion of the pair of legs of a body segment. The name Diplopod^ 

 adopted by Gervais and some other authors, has the advantage of 

 having thus a dynamical value. 



The multiplicative method is, in general, a degradational one. 

 When it affects only subordinate parts of the structure, as the 

 length of the tail of Mammals, or of Reptiles, &c., the forms are 

 not necessarily degradational. But when it affects the general 

 structure, and the types are indefinite in segments, like the 

 Myriapods, Worms, and Snakes, the forms are degradational. 

 In Mammals, the tail may be said to have indefiniteness of limit ; 

 but, since this part is only an appendage to the body and has 

 little functional importance, its elongation cannot properly be re- 

 garded as a mark of degradation, although one of inferiority. 



