82 



On the Clmsification of Animals 



anterior in position to the normal locomotive organs ; the poste- 

 rior portion is the rest of the structure. The anterior is emi- 

 nently the cephalic portion. The digestive viscera from the 

 stomach backward, and the reproductive viscera, belong as char- 

 acteristically to the posterior portion. 



It follows, further, from the cephalized nature of an animal, 

 that its primary centre offorce^ or the point from which concentra- 

 tion and the reverse are to be measured, anteriorly and poste- 

 riorly, is in the head, near the anterior extremity of the structure. 

 In an Insect or Crustacean, its position is between the n^outh and 

 the organs of the senses — ov^r which part the cephalic mass is 

 located. This is sustained by embryogeny ; and also by the fact, 

 that, as the two- most fundamental characteristics of an animal 

 are its being sense -bearing and mouth-feeding, the mouth, on de- 

 scending to the simplest of animals, is the last part to become 

 obsolescent. Only in the inferior Invertebrates is the position of 

 the mouth approximately central in the structure, as explained on 

 page 90.* 



1. Methods of Cephalization. 



The methods, according to which the grades of cephalization 

 are exhibited, may be arranged under the following heads : 



A. Size {force-measured) of life-system : each type, between 

 Man at one extreme and Protozoans at the other, having its spe- 

 cial range of variation in this respect. 



B. Functional: or variations as to the distribution of the 

 functions anteriorly and posteriorly , and as to their condition. 



C. Incremental : or variations as to vegetative increment, that 

 is, as to amplitude, and multiplicative development. 



D. Structural : or variations in the conditions of the structure, — 

 whether (1) compacted, or, on the other liand, resolved into nor- 

 mal elements ; (2) simple, or complex by specialization ; (3) de- 

 fective, or perfect ; (4) animal-like, or plant-like. 



E. Postural : or variations as to posture. (Only in Verte- 

 brates.) 



F. Emhryological : or variations connected with the develop- 

 ment of the young. 



G. Geographical distribution. 



For greater convenience and uniformity, the methods under 

 these heads are mentioned beyond as they appear when viewed 

 along the descending line of grade, instead of the ascending, 

 This is, in fact, the more natural way, since the typical form in a 



Tlipr^ r>-iny nlso be one or more spcondary centres of force ; but they are, 

 as regards the subject before us, of comparatively small importance. The in- 

 dependent development of the abdomen and cephalothorax in Crustaceans is 

 a case of the kind. 



