170 A. B. DAWSON 



study was made, the gland cells are predominant in the epider- 

 mis, and it does not appear necessary to regard the intraepidermal 

 fibers as being part of the afferent system. They might, it seems 

 with equal right, be regarded as part of the efferent system inner- 

 vating the epidermal effectors, the unicellular slime glands. 



SUMMARY 



1. In the earthworm, scattered nerve cells, probably vestiges 

 of the primitive nerve net of lower invertebrates, are found. 



2. At least four morphologically distinct types of cells are pres- 

 ent in certain intermuscular regions, i.e., in the circular nerve 

 rings between the longitudinal and circular muscle layers, and 

 in the peripheral nerves within the layer of circular muscles. 



3. Of the four types of cells, three, the spindle-shaped and 

 crescent-shaped bipolar cells and the triangular tripolar cells, are 

 believed to be associated with the effector portion of the nervous 

 system and to represent outh^ing cells which, in the phylogenetic 

 development of the central nervous system, have not been incor- 

 porated in the ventral cord. 



4. The fourth type, the long slender pyramidal or spindle- 

 shaped cells, are contained almost entirely within the circular 

 muscle layer, but also send fine processes into the epidermis. In 

 structure, staining properties, and their relation to the ventral 

 cord, they resemble the cells of the epidermal sense organs and 

 have been interpreted as deep-lying sensory cells. 



