GENERAL SUMMARY. 211 



11. The transfer of functional centers always takes place in a cephalo-caudal 

 direction. The process consists in the progressive elimination of muscular, 

 excretory, nutritive, and structural tissues from the anterior end of the head, and 

 the corresponding increase of the same structures at some point farther back, the 

 degree of elimination varying, in the main, with the linear and lateral location 

 of the parts concerned. On the other hand, the primary sense organs, visual, 

 gustatory, and olfactory, and their cerebral centers, never shift their relative 

 positions, and steadily increase in volume and structural detail. 



Hence there are three factors that determine the character of the anterior 

 end of the body; i. the progressive elimination of the more lateral, non-sensory 

 parts; 2. the increasing development of the more axial sensory and nervous ones; 

 and 3. the establishment of nervous continuity between the old nerve centers at 

 the anterior end of the body and the new ones at the posterior end as fast as the 

 latter are formed. Hence the anterior end of the body throughout the arachnid- 

 vertebrate stock tends to become more and more sensory, coordinating, and ad- 

 ministrative in character, while the posterior portion serves as the site for the more 

 modern and the more highly specialized functions. 



12. From the very earliest stages in the evolution of metamerism, the fore- 

 brain region has been devoted to vision and coordination. The first group of 

 metameres to appear behind or around the mouth (diencephalic and mesen- 

 cephalic) were devoted to locomotion and to tasting, seizing, chewing, and other 

 ingestive functions. With the increased size due to the addition of a new group 

 of metameres, respiratory and circulatory organs became essential and they made 

 their appearance behind the ingestive region. Thus the three main functional 

 divisions of the head, the visual and coordinating, the gustatory and ingestive 

 (including the primordial endocranium for the attachment of chewing muscles) 

 and the cardiac and respiratory regions were established according to the historic 

 and inherently necessary order of their evolution. They were elaborated and 

 still further emphasized by the elimination of all other tissues and organs foreign 

 to these functions. 



13. All the segmental sense organs had primarily some of the characteristics of 

 visual organs. They were located along the lateral margins of the medullary 

 plate in the fore- and midbrain regions. Throughout the entire phyllopod- 

 crustacean-arachnid-vertebrate stock two pairs of ocellar placodes are united to 

 form a true parietal eye. The retinal placodes of the parietal eye are located at 

 the dilated distal end of the vesicle ; the proximal end is usually tubular and opens 

 either on the outer surface of the head, or into the forebrain vesicle. 



Two other pairs of placodes form the stemmata, or frontal ocelli of insects, 

 or the frontal organs of phyllopods and various Crustacea. In Limulus (probably 

 also in trilobites and merostommata) they become metamorphosed into true 

 olfactory organs and represent the preliminary stage of the olfactory organs of 

 vertebrates. 



The lateral or compound eyes of arthropods belong to the most posterior 



