THE SUBDIVISIONS OF THE HEAD. II 



and by the gradual transfer of the digestive and urinogenital system still farther 

 back into the newly developing trunk. 



We need not follow in detail the further progress of these changes in the 

 higher vertebrates; the atrophy of the gills and the development of the lungs 

 behind them; the atrophy of head kidneys, and the development of new ones 

 farther, and then again farther back; and the final shifting of locomotor func- 

 tions to the pelvic appendages, are all familiar manifestations of the same 

 process. 



Thus the evolution of the arthropod-vertebrate stock consists: i. in the 

 successive generation of groups of like metameres, each group being from the 

 beginning somewhat different from the preceding one; 2. in the subsequent en- 

 largement, diminution, or elimination of segmental organs and the consequent re- 

 adjustments that follow these changes; the result always leading toward a more 

 successful linear coordination of unlike organs, the process attaining its highest 

 expression in man. Hence, broadly speaking, the progress of organic evolution 

 in segmented animals may be measured by the extent to which the linear coordi- 

 nation of unlike organs replaces the linear succession of like metameres. 



IV. THE SUBDIVISIONS OF THE INCIPIENT VERTEBRATE HEAD IN 



THE ARACHNIDS. 



The five main divisions of the anterior part of the body in the arachnids are 

 as follows: (Figs. 3, 5, 14-21.) i. The procephalon, or primitive head, consists 

 of three pre-oral segments, the principal organs contained in it being the rostrum, 

 olfactory lobes, cerebral hemispheres, the visual and the olfactory organs. 2. 

 The dicephalon consists of two or three metameres immediately surrounding the 

 mouth, and includes the stomodaeum with its appropriate nerve centers, the leg- 

 jaws, principal gustatory organs, and the anterior part of the endocranium. 3. 

 The mesocephalon consists of three or four posterior thoracic metameres and in- 

 cludes the principal locomotor appendages, auditory and excretory organs, and 

 the posterior part of the endocranium. 4. The metacephalon, or vagus region, 

 consists of from two to four greatly modified metameres, the appendages being 

 either very small and standing close to the median line, or absent, or converted 

 into sense organs. The neuromeres and their ganglia are large, but very compact. 

 Other components of the metameres are absent, or small and degenerate. The 

 whole region forms a highly specialized, constricted intermediate zone lying be- 

 tween the mesocephalon and the next following division. 5. The branch io- 

 cephalon consists of four or five metameres, in which are located the principal 

 respiratory organs, branchial cartilages, and the heart. 



The Brain. The structure and grouping of the neuromeres reflect the condi- 

 tions characteristic of these subdivisions of the body, thus laying the foundations 

 for the subdivisions of the brain in vertebrates. In the latter, the original appearance 



