LEWIS. — POLYCH^TE ANNELIDS, 227 



In spite of the great amount of productive work which has been clone 

 in recent years on nerve anatomy and histology, many of the prominent 

 questions connected with nervous structures are still matters of dispute. 

 The neuron theory, the question of the restriction of medullated nerve 

 fibres to vertebrates, the nature and function of giant fibres in inverte- 

 brates, the relation of sense organs to nerve fibres, and, very recently, 

 the presence and meaning of a centrosome in ganglionic cells, — all these 

 are questions of a broad and fundamental nature, and every observation, 

 however limited, which bears directly upon the solution of any of these 

 questions is of value to those engaged in the study of nervous structures. 

 A very few weeks of study upon the nervous system of the two annelids 

 named demonstrated that they were peculiarly interesting in relation to 

 some of these particular questions ; for this reason my work has been 

 restricted to observations bearing upon these questions of neurology. 



i 



II. CENTRAL NERVOUS SYSTEM. 



The topics treated under the Central Nervous System fall naturally 

 into two divisions, The Leydig's Fibres, and The Giant Cells. But for 

 a correct understanding of these structures a short general account of the 

 nervous system will be necessary. 



1. General Topography. 



a. Location of Nervous System. 



The whole nervous system in these annelids seems to present a very 

 rudimentary condition. With the exception of the brain, it lies in the 

 hypodermis throughout the whole length of the animal. At no region 

 except the brain is it found internal to the circular muscles. The gan- 

 glionic cells are always distinguishable from the adjacent cells of the 

 hypodermis by their form and nuclei ; but on the ventral side of the 

 nerve cord there is no membrane nor other boundary separating them 

 from the adj.-icent cells of the hypodermis (Plate 1, Fig. 5, Plate 3, 

 Fig. 21). On the dorsal surface of the nerve cord, however, and on 

 the dorsal parts of its lateral edges, such a boundary is present. The 

 circular muscles on the dorsal side of the cord are everywhere strongly 

 developed, and the lateral nerves pass off ventral to these muscles. 



b. Distribution of Ganglion Cells. 



There is no evidence of any strictly segmental arrangement of the 

 parts of the central nervous system, either in the grouping of cells into 



