THE METENCEPHALON AND THE BARNCHIENCEPHALON. 



69 



head, from time to time, thus leading to the union of the entire group of 

 branchial neuromeres with the posterior portion of the brain. 



We therefore look on the hindbrain of vertebrates as composed of two main 

 parts successively added to the midbrain, namely: a. the vagus section, consisting 

 of from two to four fused neuromeres, associated with appendages that have under- 



OUactory 



olfactory organ 



Coordination 



hemispberea 



Visual 



parietal eye 

 lateral eye 



Swfallow^ing 



stomodaeum 

 Infundibulum 



Chewing 

 Gustatory 



leg-jav's 

 oral- arches 



Locomotor 

 Auditory 

 Equilibrium 

 Vagus 



tactile 

 lateral line 



Respiratory 



cardiac 

 branchial 

 hysobrancblal 

 aympalbetic 



Digestive 



excretory 

 genital 



Fig. 57. — Semi -diagrammatic figure of an arachnid brain and nerve cord. It represents, in the main, the 

 conditions in Limulus. The parietal eye and the vagus lobes and nerves, however, have the location and arrange- 

 ment characteristic of the scorpions. 



Fig. 58. — Here the same parts are shown in the position they are supposed to have in vertebrates. The prin- 

 cipal changes are in the position of the optic ganglia, which have moved backward and upward to form the 

 optic lobes; the pedal ganglia have become the cranial ganglia of the fiith, seventh, eighth, ninth, and tenth nerves; 

 the coxal gustatory organs lay the foundations for the principal lines of canal organs. The branchial neuromeres 

 have fused with the brain, and the components of the several branchial and vagus nerves going to the same kind of 

 organ have united, forming compound nerves distributed respectively to the respiratory organs, sense organs, 

 heart, intestine, and the great branchio-thoracic, or hypo-branchial muscles. The similiarity in the linear distribu- 

 tion of the principal functions, or the nerve centers controlling them, is shown by the inscriptions between the 

 figures. 



gone extensive reduction and modification, the most important remnants forming 

 organs of a tactile or gustatory nature; this characteristic group of neuromeres 

 is largely sensory, and now forms the hindbrain, or metencephalon of arachnids; 

 and h. a second group of neuromeres belonging to the branchial segments, and 



