228 PROCEEDINGS OF THE AMERICAN ACADEMY. 



ganglia, or in the relationships of the nerves given off from the ventral 

 cord. Ganglionic cells are found abundantly throughout the length of 

 the central nervous system, but, while more numerous in some regions 

 than in others, they show no metameric plan of arrangement, nor any 

 definite grouping into ganglia. 



c. Number and Distribution of Nerves. 



In each body segment lateral nerves are given off from the ventral 

 cord in very large numbers. In some segments I have demonstrated 

 more than thirty-five on each side of the body. The only feature of 

 these nerves in any way suggestive of a metameric arrangement is their 

 greater size in the region of the parapodia. These nerves are without a 

 sheath, branch freely, and anastomose with one another, so that beneath 

 the hypodermis there is a network of nerve fibres which forms an almost 

 continuous sheet (Plate 1, Fig. 4). Figures &% and 67 (Plate 8) are 

 diagrams obtained by superimposing in each case a series of frontal 

 sections of C. producta. Each shows the number and arrangement of 

 nerves and giant cells in the anterior part of one segment. Figure 21 

 (Plate 3) is a typical cross section of the nerve cord (from C. producta) ; 

 it shows that the ganglionic cells occupy the ventral and lateral portions 

 of the cord, and that on the dorsal side of the cord there is the cross sec- 

 tion of what appears to be a tube. This is Leydig's fibre, which lies just 

 ventral to the circular muscles. 



In general, these facts point to an undifferentiated condition of the 

 nervous system. Whether this condition is primitive, or has arisen sec- 

 ondarily, and therefore represents a degraded state of the nervous 

 system, can of course be settled only by the aid of phylogenetic as well 

 as ontogenetic evidence. 



2. Methods. 



My studies on the central nervous system of these annelids had to be 

 pursued almost entirely upon sections of hardened material. Attempts 

 to isolate the nerve cord were very unsatisfactory, because of the close 

 intimacy of nervous system and hypodermis. Several methods of fixing 

 and staining were employed, but of these the one given by vora Rath 

 ('93, p. 102) proved to be pre-eminently valuable for the study of 

 every part of the nervous system. 



To prevent undue contraction it was found necessary to narcotize the 

 worms thoroughly in all cases before killing. For the nervous system, 

 owing to its situation in the hypodermis, partakes of all the folds of the 



