TETHYS CERVINA DALL AND SIMPSON 45 



ones, which ramify throughout the liver, conveying its secretion 

 to the central biliary cavity. Into the posterior side of this 

 chamber opens the hepatic coecum (PI. VI, fig. 32, h. cce.), a 

 narrow curved cylindrical tube, at first imbedded in the liver, 

 but appearing at its surface for the distal third of its length. It 

 is 9.0 mm. in length, the diameter varying from i.o mm. to 1.25 

 mm. It is traversed by two longitudinal folds, arising from 

 opposite sides, the one higher than the other, which meet and 

 overlap, thus dividing the lumen into two practically separate, 

 longitudinal portions, united at the blind, distal end. At the 

 opening of the coecum into the bile chamber, the anterior one of 

 these communicates freely with the latter, its walls being grooved 

 and folded in prolongation of similar folds and grooves in the 

 wall of the bile chamber. The posterior half of the coecum 

 communicates as a deep groove with the intestine, one of the 

 median, longitudinal folds in the wall of the coecum being 

 continued across the opening of the bile chamber and down into 

 the intestine (PI. VI, fig. 32, /. r.), there gradually merging with 

 its wall. 



THE CENTRAL NERVOUS SYSTEM. 



The central nervous system (PI. VII, fig. 34) is made up of 

 three pairs of ganglia resting upon the posterior end of the phar- 

 yngeal bulb, and, with their commissures, encircling the com- 

 mencement of the esophagus. These are the cerebral, the pedal 

 and the pleural ganglia, each pair united by commissures of 

 varying length, while the ganglia of each side are united in a 

 triangular grouping by the cerebro-pedal, the cerebro-pleural, 

 and the pleuro-pedal connectives. Close to these ganglia, and to 

 be included with them in the central nervous system, are the 

 buccal ganglia, situated on the ventral side of the esophagus, and 

 forming with their cerebral connections, the cerebro-buccal con- 

 nectives, another ring around the anterior end of the alimentary 

 canal. These structures will be taken up briefly in the following 

 description. On plate VII is figured a dorsal view of the whole 

 central nervous system of Tethys cervina, excepting the buccal 

 ganglia, together with the origins of the nerves taken up in the 

 following pages. As in the similar figure of the preceding species 

 of Tethys, given on Plate II, the nerves are numbered in the 



