MUSEUM OF COMPARATIVE ZOOLOGY. 167 
the brain, and these groups of fibres are apparent merely by virtue of a 
different refractive power. They are somewhat irregularly arranged, 
and yet correspond very nearly on the two sides of the body. I regard 
them as groups of nerve fibres. They may be seen to turn dorsad 
(Plate VII. Fig. 92, x. a.) in the tissue of the wall, and probably inner- 
vate the numerous sensory cells found in this wall. 
b. Ventral Nerve Cord. 
The ventral nerve cord extends directly posteriad from the brain 
through the entire body. It is located in the ventral line just above 
the epithelial layer, and appears in cross sections near the middle of 
its length as a roughly cordiform mass (Plate VIII. Fig. 101), which 
is separated by internal divisions into three areas. These areas repre- 
sent the three nerves of which the cord is composed. The median 
area (Plate VIII. Fig. 101, 2. m.) is triangular, with its apex directed 
ventrad, and is, so to speak, wedged in between the two oval lateral 
areas. Near the brain the form of the ventral nerve cord is some- 
what different, and gives a hint as to its relation to the brain, which 
can be easily traced in any series of sections which includes the brain 
and the following portion of the body. The first trace of a partition 
in the fibrous mass is found well forward in the brain, and is shown in 
the arrangement of the small nervous nuclei, and of the dorsoventral 
groups of nerve fibres already mentioned ; these indicate a division of 
the brain into a central mass quadrangular in cross section, and two 
lateral masses more or less rounded off on the outer side (Plate VI. 
Fig. 86). At the position of the fourth pair of large cells a row of 
small cells, already mentioned, makes this division more apparent, and 
even before reaching the ventral nerve cord one sees the separation of 
the three portions by fibrous bands which cross the brain vertically. At 
the beginning of the nerve cord the three portions are of about equal 
size; gradually the lateral areas push themselves in under the central 
portion until the latter has been compressed into a triangular shape, 
with the lateral areas almost touching in the median plane beneath it. 
This relation, with slight modifications, is preserved throughout the en- 
tire length of the animal, and I do not find, as Biirger (p. 641) has 
maintained, that the median portion is more prominent in the anal 
ganglion (Plate VIII. Fig. 96, x. m.). The central and lateral portions 
seem to be, so far as I can find, alike in structure. The number of 
faintly stained homogeneous processes in the three portions is nearly 
equal ; in the posterior part of the body they are perhaps more numer- 
