Pomona College, Claremont, California 



77 



Along slightly different lines the brain may be divided into sensory, 

 cross-connective visceral and nucha] parts. 



Sensory System. The prostomial system oi sense organs in this 

 form is one of the most complex and highly specialized among 

 annelids. Just forward of the central cross-connective part (Fig. 

 6) the brain divides into two lateral halves, which extend down 

 and conned with the two front branches oi the visceral ganglion. 

 These halves (Fig. 7), give ofl two rounded lateral sensors lobes 

 (Figs. 1,4, 6; b) and then divide into quadrants, the larger ol which 

 compose the lower pair. Each ol these lower quadrants subdivides 

 into lour ami sometimes five lobes (Figs. I. 4, 5; c) . I he inner 

 three are long anil slender, while the outer odc, which shows a 

 tendency in large specimens to subdivide at the tip is much shorter 

 and broader. The dorsal pair ol quadrants each divide mto three 

 distinctly longer and more slender lobes, giving in all from fourteen 

 to sixteen lobes. The lobes are each composed ol a cellular and a 

 fibrilar tract. The liber bundle is iiw the inside and runs directly 

 back ii) the main brain, while the cellular area is on the surface sub' 

 of each lobe and is directly connected with the subcuticular sense 

 organs. These cells (Fig. 1_; u), underlie the whole ol the pros- 

 tomial cuticle ami are connected wit!) the brain by means ol libers 

 which run into the brain in larger ov smaller irregularly placed 

 bundles or even as individual libers, threaded between lite epithelial 

 cells of the subcuticular region. The whole ol tin- prostomial ner- 

 vous sNstcin, including the visceral ganglion and its branches give 

 ott sensory libers in great abundance. In main cases the sense cells 

 seem to send ott sensory libers direct to the cuticle. 



In the front lobular region, besides the sense cells and the ordi- 

 nary small nerve cells (Figs. 5, 6, 7, 10; d), there are a lew large 

 cells embedded in the brain (Figs. 5, 10; I). These have nucleoli 

 anil in some cases libers can be traced from them. They are much 

 smaller than the giant cells (Figs. II, S ; g) in the ventral nerve 

 a^\\, more irregular, the structure ol the protoplasm is much finer 

 and they arc much harder to stain with ordinary stains. Hema- 

 toxylin leaves them clear unless a mordant is used. Mcthylcn blue 

 and the other common nuclear stains >.\o not touch them. \ illam's 

 copper hematoxylin gives the best results, staining the protoplasm 



