THE NERVOUS SYSTEM. 



451 



It sends nerves to three sets of cephalic sense organs, to the stomodaeum, 

 mantle, and to the oral appendages and their follicles. It is connected with the 

 ventral ganglion by double circumoesophageal commissures, and probably includes 

 the rudiments of the primitive forebrain and one or more segments of the dien- 



J-br. 



pu 



FIG. 306. Figures illustrating the structure and development of Sagitta. (In part, after Hertwig, Grassi, and 



Epatiewsky.) Semi-diagrammatic. 



cephalon. In its general appearance, it resembles the brain of a primitive arthro- 

 pod more than that of any other invertebrate. This is shown by the internal 

 arrangement of the commissural fibers, medullary substance, and nerve cells, as 

 well as by the arrangement of the principal sensory nerves, and the distribution 

 of the voluminous stomodaeal nerves and ganglia. The lateral stomodaeal ganglia, 



