MUSEUM OF COMPARATIVE ZOOLOGY. 159 
5. Nervous SYSTEM. 
a. Brain. 
The anterior ganglionic mass, or brain, forms the larger portion 
of the floor of the anterior chamber (Plate V. Fig. 63). In general 
it is somewhat wider than long, being from 0.16 to 0.28 mm. in width, 
and from 0.12 to 0.2mm. in length, and has an average thickness of 
only 0.14 mm. (Figs. 3, 8, 63, 72-88). Its anterior limit is the one 
most difficult to make out, since the brain substance goes over gradually 
into the tissue in front of it, from which it is not separated by any 
prominent capsule. The middle of the dorsal surface is marked by 
a longitudinal groove, in which the csophagus lies. Behind the nar- 
row meagre cerebral commissure the csophagus is separated from the 
brain by a considerable space, and here the dorsal groove in the lat- 
ter is wider and less defined than farther forward. (Cf. Plate VI. Figs. 
72-88.) Laterally the limits of the ganglionic mass are more distinct, 
although no envelope of connective tissue separates it plainly from the 
adjacent cells, In fact, there does not seem to be a definite capsule 
anywhere, even on the dorsal surface. In places the limits of the mass 
are so sharp as to suggest a covering membrane, but I was unable to 
find any corresponding nuclei. The counective-tissue fibres which bound 
the ventral nerve cord dorsally are first apparent behind the last pair of 
large ganglion cells of the brain. 
a. Ganglion Cells. — On the whole the brain is poorly supplied with 
ganglion cells; of those found, one can nevertheless distinguish two 
kinds, which represent extremes in size. The first and smaller kind is 
only moderately abundant, but they far exceed in number the second. 
No appreciable amount of cell protoplasm ean be seen about them, but 
they appear everywhere simply as small oval nuclei (Plate V. Fig. 68) 
only 4-5 a in diameter. These nuclei stain deeply, and show a thick 
nuclear membrane with numerous chromatic granules, of which one, 
or occasionally two, are very prominent. In general they correspond 
closely to the nervous nuclei (Nervenkerne) described by various ob- 
servers for different groups of animals. A further point of resemblance 
is found in their position, for they lie embedded in a mass of fibres, 
and, although it is difficult to decide whether certain of the fibres are 
connected with them, appearances decidedly favor this view. Cells of 
this kind are most abundant on the anterior face of the ganglionic mass, 
and around the stalks of the dorsal cells. In the fibrous mass of the 
brain they occur ordinarily only at the ventral surface, and in one or 
