No. 3.] STUDY OF STENOSTOMA LEUCOPS O. SCHM. 285 



Nervous System. 



Brain. — In a living worm as seen from the dorsal or ventral 

 surface the brain may be seen to lie immediately in front of the 

 oral end of the pharynx. By focusing upon the living worm 

 it may be seen to lie in a plane dorsal to the pharynx and 

 to be surrounded by the parenchyme, which appears to support 

 it and hold it in place. The brain consists of two large 

 ganglia lying one on either side of the median line with 

 their posterior ends connected by a large transverse commis- 

 sure. 



In a horizontal optical section each of these ganglia is seen to 

 consist of a large oval posterior part and a smaller blunt forward 

 projection, the anterior end of which is excavated for the recep- 

 tion of the ciliated pits. As may be seen in both longitudinal 

 and cross-sections, the two ganglia lie in the same horizontal 

 plane. 



When the ganglia of a quiescent worm are observed from 

 above (Fig. 16), they are seen to be but 8 /x apart at their ante- 

 rior ends, while they are 25 /x apart at their posterior ends, thus 

 making the longitudinal axes of these lobes stand at an angle of 

 about 30° with one another. The longitudinal axis of each of 

 these ganglia is 50 /x. The greatest transverse axis is 25 yu. and 

 the greatest dorso-ventral axis is 30 /li. 



In sections the ganglia may be seen to be made up of branched 

 cells which may be seen in many places to be connected by their 

 processes (Figs. 17, 18, and 19). Thus in Fig. 19, which repre- 

 sents a cross-section of the anterior part of the brain ganglion 

 taken along the line ab in Fig. 16, the ganglion consists entirely 

 of branched cells. In Fig. 17, which represents one-half of a 

 cross-section of the posterior end of the brain taken along the 

 line cd in Fig. 16, the ganglion consists of an outer thick layer 

 of branched cells and a central mass of parallel fibres which 

 are continuous with the fibres of the commissure. In Fig. 18, 

 which represents a horizontal section of the brain, the part of 

 the gangha just anterior to the commissure consists entirely of 

 branched cells. 



These branched cells vary much in size as well as in form. 

 The smallest are scarcely more than i jx in diameter, while 

 the largest are 4.5 yu, in diameter. These cells may be seen in 



