TETHYS CERVINA BALL AND SIMPSON 51 



to the peritoneum and the muscles of the dorsal body wall, just 

 above and behind the region of the central nervous system. 



The second pleural nerve (pi. 2), arises just exterior to the 

 origin of the pleuro-visceral connective. It passes backward as 

 a long slender unbranched trunk in the dorsal peritoneum, to the 

 region of the pericardium, in the anterior wall of which it ramifies 

 among the muscles. 



From the right pleural ganglion but one nerve (pi. i}, is 

 given off. It corresponds to the first one of the left side and has 

 a similar distribution. It also receives an anastomosing branch, 

 30,, from the third pedal nerve of the right side. 



From the median posterior face of the ganglia arise the long 

 and strong connectives, which pass backward to the ganglion com- 

 plex upon the visceral loop, situated immediately below the 

 anterior boundary of the pericardium. The left of these connect- 

 ives (pi. v. con.}, is slightly longer than its fellow, measuring 

 14.0 mm., as compared with 12.0 mm. for that of the right side. 

 The right of these (pi. par. con.}, is the pleuro-parietal connect- 

 ive, the left the pleuro-visceral one. Their peripheral relations 

 will be taken up further on. 



Buccal ganglia. The buccal ganglia are oval in outline, 

 each measuring 0.45 mm. in length by 0.42 mm. in width, and 

 are connected by a broad commissure 0.18 mm. in length, so that 

 the two ganglia are distinctly separated from each other, though 

 enveloped in a common connective tissue sheath. From the 

 anterior median face is given off a strong unpaired nerve as in 

 Tethys dactylomela, soon bifurcating to the muscles of the rotella. 

 From the outer side of each ganglion four nerves are given off, 

 in addition to the cerebro-buccal commissures. These nerves are 

 distributed to the pharyngeal bulb, the salivary glands and the 

 esophagus, but their ramifications were not worked out in detail. 



Parieto-visceral ganglia. The parieto-visceral ganglion 

 group (PI. VIII, fig. 35, par. v. g.), is situated beneath the dorsal 

 wall of the body, slightly to the right of the median line and 

 directly below the anterior border of the pericardium. The com- 

 position of the group as made up of a right and left portion, 

 fused in the median plane, can be readily made out, but any 

 further division into component ganglia is not indicated in surface 

 view. The double nature of the group is marked only at the 



