20 



OUTLINE OF THE ARACHNID THEORY. 



spending trunk muscles and the absence of appendages, or their conversion into 

 sense organs. In them is located an important decussation of the longitudinal 

 tracts passing from the cord to the brain, and vice versa; and the vagus neuro- 

 meres are the most anterior ones in which such a crossing takes place. (Figs. 



65, 66, 114, v. dec.) 



The vagus nerves have special relations with the heart, intestine, and integu- 

 ment. They are the only segmental nerves that are persistently directed back- 

 ward into foreign territory, a result that is due in part to the forward concentration 

 of the vagus neuromeres, and in part to the backward growth of the nerves and 

 the atrophy of their native metameres. (Figs. 38, 42, 57, 70, 71.) 



FIG. 14.- Diagram of an insect embryo (Acilius) in FIG. 15. Scorpion embryos in mercator 



mercator projection. projection. 



The interpolation of the vagus neuromeres between the mesencephalon and 

 the branchiencephalon is a very important and striking feature in the morphology 

 of the arachnids. They form a compact group, or distinct brain region, which in 

 its anatomical and physiological characters, and in the distribution of its nerves, 

 is very similar to the vagus region of vertebrates. 



5. The Branchiocephalon. 



This group of metameres, four or five in number, is the least specialized of 

 any so far considered. The appendages may be well-developed (Limulus and 

 many Crustacea) or they may be rudimentary. In the higher forms, this region 

 is chiefly notable as the site of the respiratory organs, i.e., the tracheae, gills, lung- 

 books, and heart. (Fig. 3, br.c.) 



The mesoderm is complete, each metamere containing well developed somites 



