No. 2.] NOTES ON AEOLOSOMA TENEBRARUM. 183 



characteristics, although Vejdovsky favored classifying it with 

 the Turbellaria on account of the similarities which he found 

 between the ciliated pits, muscle plates, and gland cells of these 

 forms, together with the structure of the brain and the sup- 

 posed lack of a ventral cord. 



The course of the large nerves running back from the brain 

 has not yet been traced for an annelid ; they present an anom- 

 alous feature which is most interesting from its suggesting a 

 possible transitional form of nervous system between unseg- 

 mented and segmented worms. The position of the brain in 

 the first segment, the continuity of the entire nervous system 

 with the epidermis, and the wide separation of the halves of 

 the ventral cord are primitive characteristics which would be 

 consistent with such a form. 



REFERENCE LETTERS. 



ag. accessory ganglion. nib. membrane lining the body cavity 



b. brain. and covering the free, lower 



c. connecting commissures of the surface of the brain. 



ventral nerve cord. vif. muscle fibers attached to the 



cl. cluster of large sense organs in brain and connecting it with the 



the end of the prostomium. epidermis of the ventral side. 



d. second ganglion of the oesopha- n. nucleus. 



geal commissure. '^1-3 first, second, and third pairs of 



ep. epidermal cells. cephalic nerves. 



/ lateral ciliated furrows leading to 7wi. nuclei of the lining membrane. 



the mouth. o. refractive body. 



g. gland cells. oc. oesophageal commissure. 



/. lip bordering the mouth and the P. posterior ganghonic lobes. 



ciUated furrows. ». vesicle containing a refractive 



///. lateral nerves. body. 



m. mouth. vc. ventral nerve cord. 



