OF ARTS AND SCIENCES. 103 



tiou it is roughly elliptical, or bluntly crescent-shaped, with the con- 

 cavity up (PI. II. figs. 16, 17 ; PI. III. figs. 22-24). Its central part 

 is fibrous, and has a median superior and a median inferior groove. 

 Lying in the floor of the superior groove are the thi-ee " primitive 

 nerve fibres " of Ratzel (" Rohrenfasern ") which are claimed by 

 Bulow ('83, p. 92) to be of mesodermic origin, and therefore not to 

 be compared to the chorda dorsalis of vertebrates. A single "fibre" 

 begins in the third segment, while farther back two smaller ones are 

 added to it, one on each side. The three, continuing side by side 

 without branching, remain of nearly uniform calibre until just before 

 the posterior end of the ventral cord is reached, when they disappear. 

 Through most of their course they appear to be simple empty tubes, 

 but the middle and larger " fibre " shows slightly stainable contents in 

 its extreme anterior and posterior ends. The inferior groove is filled 

 throughout its course with ganglionic nerve cells, and this is the only 

 part of the cord where the layer of nerve cells is unbroken. The 

 fibrous central part of the cord swells slightly and gradually in the 

 middle of each segment (PI. I. fig. 10). Its upper surface is free from 

 nerve cells throughout its entire length, whereas the rest of its surface 

 is covered by a layer of them which varies from one to four cells in 

 depth, and forms a series of ganglionic swellings. This ganglionic 

 layer is thickest at the centre of the segment, where the fibrous part 

 itself attains its greatest diameter. There is a very short space in the 

 region of the partitions, where the ganglionic cells, except in the ven- 

 tral groove, are wanting. Muscular fibres pass from the centre of 

 each ganglion to the body walls on each side and to the alimentary 

 canal above. The ganglionic matter of the first four bristle-bearing 

 segments is fused together into one mass, with only a slight increase in 

 size in the middle of each segment. 



The ganglionic nerve cells are apolar, unipolar, or rarely bipolar, 

 and polygonal in form. Their nuclei are very large, leaving only a 

 narrow zone of cell protoplasm around them. In each nucleus there 

 may be one or several nucleoli, which appear as bright granules. The 

 processes of these cells are sometimes traceable for a long distance, 

 and pass into the fibrous nerve substance. 



The nervous system is invested by a neurilemma formed of connect- 

 ive-tissue cells, which also pass in between the ganglionic and fibrous 

 elements, and separate them from one another. This separation is, 

 however, not complete, nor is it always evident, though, for the sake 

 of uniformity, it has been so represented in the figures. 



The fibrous part of the nervous system is composed of fibres, each 



