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, n. a.) in the tissue of the wall, and probably inner- 

 vate the numerous sensory cells found in this wall. 



h. Ventral Nerve Cord. 

 The ventral nerve cord extends directly postei'iad 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, n. 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 Burger (p. 641) has 

 maintained, that the median portion is more prominent in the anal 

 ganglion (Plate VIII. Fig. 96, n. 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- 



