No. 2.] NOTES ON AEOLOSOMA TENEBRARUM. i8l 



Commissure. — Immediately after leaving the brain the oeso- 

 phageal commissure passes into an accessory ganglion, from 

 which a nerve runs forward into the prostomium (PI. XXI, 

 Fig. i). It then passes downward and backward, in close con- 

 nection with the epidermal wall, to the ventral side, where it 

 expands into a second ganglion before passing into the ventral 

 cord (PI. XXI, Fig. i). The fibers of the commissure form a 

 broad band which is clearly distinguishable, but it is often 

 difficult to determine whether the cells along its course belong 

 to it or to the epidermis. 



Ventral Cord. — The two parts of the ventral cord are sepa- 

 rated by about one-fifth of the diameter of the body and com- 

 municate with each other by fibrous commissures, forming the 

 ladder type of nervous system (PI. XXI, Figs, i, 12). 



There is one pair of ganglia in each segment, and each 

 ganglion is deeply bilobed, the anterior lobe being somewhat 

 smaller, while the posterior lobe extends out farther in the 

 body wall. The fibrous portion forms the greater part of the 

 ganglion, and is covered by cells one layer deep (PI. XXI, Fig. 

 II). In the posterior segments the ganglia are crowded together 

 more closely than in the anterior segments (PI. XXI, Fig. i). 



Lateral Nerves. — Four distinct lateral nerves are given off 

 from each ganglion, two from the anterior and two from the 

 posterior half (PI. XXI, Figs, i, 11). 



This whole system of brain, ventral nerve cord, commissures, 

 and nerves is connected throughout with the epidermal wall, 

 no portion of it being entirely free in the body cavity. The 

 cells of the ventral ganglia, as well as those of the brain, are 

 often so closely connected with the epidermis that it is hard to 

 find the boundary line between them. The nuclei of the gan- 

 glion cells are of about the same size as those of the epidermis, 

 but stain a little more deeply. 



Ciliated Pits. — Vejdovsky^ states that in Aeolosoma we 

 find the only instance of an oligochaete possessing a pair of 

 lateral ciliated pits, and he compares them with the ciliated 

 pits of the Turbellaria. From the dorsal side the appearance 

 is very similar to these organs in the Turbellaria, but frontal 



1 Vejdovsky, F. Thierische Organismen der Brunnenwasser von Prag. 1882. 



