104 PROCEEDINGS OF THE ACADEMY OF [Feb., 



also to be seen just below the brain, on its ventral side, and extending 

 out toward the hypodermis. The presence of this group is, however, 

 apparently unusual, since it could not be found in other examples. 



The ganglion cells of the brain are all very similar in size and form, 

 and have the appearance of being multipolar, axons and dendrites not 

 being distinguishable in most cases. Some of the ganglion cells are, 

 however, demonstrably bipolar, as, for example, the sensory neurons 

 of the tactile cilia. It is probable that all the ganglion cells are of 

 this type, nameh', bipolar. The branching processes (dendrites) form a 

 loose meshwork; some extending peripherally between the hypodermal 

 cells; some — probably axons — extend centrally to enter the neuropil 

 mass. In figs. 19 and 20 many of the ganglion cells appear as if 

 seated on tiny papillse of the neuropil. These papilla-like projections 

 are, of course, only the central processes of the ganglion cells themselves 

 which enter the neuropil and there divide into fil3rillae. 



The cytoplasm of the ganglion cells, in contrast to that of the neigh- 

 boring supporting cells of the hypodermis, stains rather deeply in iron- 

 hsematoxylin. On examination under a high power it presents a 

 granular appearance (fig. 12). The nuclei of the ganglion cells are 

 elliptical in outline and are very constant in size in the same individual, 

 measuring from 1.42 x 2.25 [j. in small individuals to 2.25 x 3.06 /.( in 

 the largest. They are rich in chromatin, which is divided into several 

 large subequal granules, none of which exceeds the others sufficiently 

 to be considered a karj'-osome, as in the nuclei of the other tissue cells. 

 This peculiarity of the nucleus of the ganglion cells renders them, in 

 this form, easily distinguishable wherever they occur. The same 

 distinction, curiously enough, occurs in HistriobdcUa (Histriodrilus), as 

 may be readily gathered from Foettinger's (1S84) figures, although 

 the author does not allude to it in the text. 



Embedded in the posterior portion of the neuropil are cells having a 

 cell-body indistinct in outline, and a large vesicular nucleus with a 

 conspicuous karyosome. These cells are similar in form and size to 

 those of the mesenchyme, which form with their branches a delicate 

 network traversing the head cavity (figs. 19, 20 and 23), and are 

 doubtless to be classed with them. 



The circumo^sophageal commissures spring from the ventro-lateral 

 portions of the neuropil, below its lateral loljes, uniting at this point 

 with the lateral portions of the transverse commissures, with whose 

 fibres they are continuous (figs. 1, 20 and 23, c.com.). Each is at this 

 point nearlj' round in section and lies in contact with the ventral hypo- 

 dermis. Scattered over its surface are a small number of gandion 



