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 0.2 mm. 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 oesophagus lies. Behind the nar- 

 row meagre cerebral commissure the oesophagus 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 connective-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 can be seen about them, but 

 they appear everywhere simply as small oval nuclei (Plate V. Fig. 68) 

 only 4-5 fi 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 fibi-es 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 



