338 THE DOLABELLINAE. 



toneum of these regions. It also forms an anastomosis with a branch of the 

 left posterior pedal nerve, 9a, in a position similar to that entered into by its 

 fellow of the opposite side. 



From the loop of the parapedal commissure, a httle to the left of its middle 

 point, a slender unpaired nerve, c, is given off. It courses backward along 

 the ventral surface of the head retractor muscles to a point below the anterior 

 end of the ingluvies, where it curves downward and backward through the con- 

 nective-tissue septum to the foot, ramifying there in the peritoneum. 



Of the above nerves the second unpaired one was described by Amaudrut 

 (1886) for Dolabella, and has been noted in various species of Tethys by Cuvier 

 (1804), Von Ihering (1877), Vayssiere (1885), MazzarelU (1893), and the writer 

 (1909). The paired nerves here described as arising from the parapedal com- 

 missure have not been found heretofore in any member of the group. It may 

 possibly be a case of a fusion of a pedal nerve with the beginning of the com- 

 missure to such an extent that it appears to be a branch of the latter on leaving 

 it, for the nerve-cells from which the fibres spring are without doubt to be 

 found in the pedal gangUon itself. 



Pleural ganglia. — The pleural ganglia rest upon the upper posterior face 

 of the pedal gangUa, to which they are united by extremely short pleuropedal 

 connectives. They are inclosed with the pedal ganglia in a close common invest- 

 ment of connective-tissue, so that they are not distinguishable until after care- 

 ful dissection and clearing. They are elhpsoidal in form, strongly flattened 

 dorso-ventrally, and about one fourth the size of the pedal gangUa, measuring 

 approximately 1.3 mm. in uddth by 1.9 mm. in length. From the upper outer 

 face of each gangUon is given off the strong cerebro-pleural connective, which, 

 at first independent, soon joins the cerebro-pedal connective and passes around 

 the oesophagus with it to join the cerebral ganglia on the dorsal side. 



Pleural nerves. — Each of these pleural ganglia gives rise to two nerves. 

 The first pair of these (Plate 2, fig. 1, pi. 1), arise from the antero-lateral face 

 of the gangUa immediately in front of the origin of the cerebro-pleural con- 

 nective. It is very delicate and passes outward, joining the third pedal nerve 

 in a common sheath and has a common distribution with it, as already given 

 above in the description of the nerves from the pedal gangUa. 



From the inner posterior border of each pleural gangUon arises a long 

 and strong connective, each passing back to unite with a peripheral group of 

 ganglia, forming the visceral loop. That of the right side is the pleuro-parietal 

 connective, pl-par. con, that of the left the pleuro-visceral connective, pl-v. con. 



