LEWIS. — POLYCH^TE ANNELIDS. 239 



In view of these facts, we are led to believe that, whether or not the 

 neuron theory holds true for vertebrates, it certainly does not apply to all 

 invertebrates. 



4. Giant Cells. 



a. General Account of Size, Number, Arrangement, etc. 



Under the name giant cells I include those large cells whose processes 

 unite to form Leydig's fibre. In both worms the most anterior of these 

 cells occupy the sub-cesophageal ganglion ; and they are found in small 

 numbers throughout the ventral cord. They are usually situated in the 

 lateral portions of the nerve cord, but sometimes lie in the ventral part 

 of it. They are not found in the brain. While in the larger number of 

 cases the connection of these cells with the Leydig's fibre could not be 

 shown in a single section, yet in several cases it has been possible to 

 do this. It is not strange, in view of the frequently curved and irregular 

 course of the cell-process, that such fortunate sections should be obtained 

 only occasionally. 



As has been stated before, these cells show no symmetry or regularity 

 of arrangement, except in the region of the sub-oesophageal ganglion. 

 In all the specimens examined, the giant cells of this region seemed to a 

 certain extent to show a paired arrangement. In no other portion of the 

 cord has anything approaching symmetry been found. 



In neither species do the different segments of the worm show agree- 

 ment in the number of these cells. By superimposing drawings of 

 serial sections of two segments, I found the number of the cells in two 

 successive segments of the nephridial region to be in one case twelve, 

 in another eight. 



The dimensions of the cells vary, and are in a measure proportional to 

 the diameter of the worm, for they increase in size from its anterior end 

 as far as that portion o^ the body in which the nephridia are found, this 

 being the thickest part of the worm, and from this region onward they 

 again diminish in size. The cells are also most numerous in the seg- 

 ments bearing nephridia, becoming scantier farther behind. But even 

 in that part of the body where the cells are largest, they vary con- 

 siderably in size. A comparison of several cells from the nephridial 

 region gave a variation of from SO^w to 52(x in the long diameter, and 

 of from 20^ to 40jiz in the short diameter. Although showing this con- 

 siderable variation in size, any of these cells are large enough to be 

 easily distinguished from other cells of the nervous system, even by the 

 use of very low powers of the microscope. 



