52 BRISTOL. [Vol. XV. 



that show in control preparations are the large bipolar cells, 

 and these are constant whatever method is employed. 



Traces of this system are found in the most anterior portions 

 of the head in the form of large bipolar cells, whose connec- 

 tions I have not yet determined. The ist ring occurs in 

 metamere IV, annulus 5 ; the 2d in metamere V, annulus 8 ; 

 the 3d in metamere VI, annulus 11, the last annulus of the 

 head region. From this point onward two rings are found in 

 each full metamere, in the 2d and 5th annuli respectively. I 

 have not found the ring behind metamere XXIV, or any well- 

 defined traces of it. 



Beginning with metamere VII, I find eight bipolar cells con- 

 necting each ring with the succeeding one, as shown in PI. V 

 and as will be described below. Each ring receives fibers from, 

 and sends fibers to, the central nervous system, and fibers 

 from the ring run to sensillae and muscles. 



Figure 18, PL VII, is a diagram of one-half of a transection 

 made in the mid-body region, designed to show the arrange- 

 ment and relations of this nerve ring. It is a projection con- 

 structed from camera drawings of the neryous elements in 

 annuli 5 and i of adjoining somites upon a single plane, 

 viewed from anterior to posterior. The anterior nerve (com- 

 pare PL V, Fig. 2) runs laterally in the ist annulus, sending 

 forward two ventral branches, between the 3d and 4th, and 

 the 4th and 5th bundles of long muscles, one to the nerve 

 ring in the 5th annulus, the other, the cut end of which is 

 shown in this figure, to a few ventral sensillae in the 4th 

 annulus. Passing out to the edge, it innervates three large 

 sensillae in annulus i, as shown. The posterior nerve rises 

 dorsally and innervates three large sensillae on the dorsal sur- 

 face of annulus i, and sends forward a branch which goes to 

 the nerve ring between the 4th and 5th bundles of dorsal 

 long muscles and on to a few dorsal sensillae in the 4th 

 annulus. The nerve ring here represented lies in the 5th 

 annulus. It consists of a complete ring of fibers which lie 

 between the layers of long muscles and the circular-oblique 

 muscle layers, whence the name I have given to it. Around 

 this ring at definite points are ten groups of bipolar cells, six 



