Pomona College, Claremont, California 77 



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

 cross-connective visceral and nuchal parts. 



Sensory System. The prostomial system of 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 connect with the two front branches of the visceral ganglion. 

 These halves (Fig. 7), give off two rounded lateral sensory lobes 

 ( P igs. 1,4, 6 ; b ) and then divide into quadrants, the larger of which 

 compose the lower pair. Each of these lower quadrants subdivides 

 into four and sometimes ti\e lobes ( Hgs. 1, 4, 5; c). The inner 

 three are long and slender, while the outer one, which shows a 

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

 and broader. The dorsal pair of quadrants each divide into three 

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

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

 Hbrilar tract. The fiber bundle is on the inside and runs directly 

 back to the main brain, while the cellular area is on the surface side 

 of each lobe and is directly connected with the subcuticular sense 

 organs. These cells (Fig. 12; u), underlie the whole of the pros- 

 tomial cuticle and are connected with the brain by means of fibers 

 which run into the brain in larger or smaller irregularly placed 

 bundles or even as individual fibers, threaded between the epithelial 

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

 vous system, including the \isceral ganglion and its branches give 

 off sensory fibers in great abundance. In many cases the sense cells 

 seem to send off sensory fibers 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 few large 

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

 and in some cases fibers can be traced from them. They are much 

 smaller than the giant cells (Figs. 11, 8; g) in the ventral nerve 

 cord, more irregular, the structure of the protoplasm is much finer 

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

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

 and the other common nuclear stains do not touch them. Villain's 

 copper hematoxylin gives the best results, staining the protoplasm 



