INTERNAL ANATOMY. 337 



paired nerve arising from the i)arapeilal coininissuro described below. It sends 

 a branch to the peritoneum of the diaphraj^m-Uke septum of connective-tissue 

 in front of the stomach, and is finally distributed to the head retractor muscle 

 near its origin. Upon the left side the relations are similar, save that no 

 branches to the diaphragm can be identified with certainty. The final termina- 

 tion is the same as that of the right side, an additional delicate branch being 

 sent to the aorta. In Plate 5, fig. 2 these relations are shown in ventral view. 



The marked asymmetry described by Amaudrut (18S6) as existing in D. 

 rumphii [= scapula], due to the large development of the third pedal nerve of 

 the right side, was not found in D. agassizi. From his brief description without 

 figures I infer that his third pedal nerve corresponds to what is here described 

 as the ninth, and which I consider to be the posterior pedal nerve. It is prac- 

 tically alike upon the two sides in the present form, that of the right side being 

 slightly stouter, but the peripheral distribution being the same. As no details 

 are given by Amaudrut of the distribution of the other pedal nerves beyond 

 slight mention of the second, I am unable to make any further comparisons of 

 the two forms. 



Parapedal commissure. — The slender parapedal commissure (Plate 5, 

 fig. 3, p-p. c.) arises from the median ventral face of each pedal ganglion, and 

 forms a loop, 12 mm. in length, which encircles the anterior aorta below the 

 pedal commissure. The point of origin of the parapedal commissure is near to 

 that of the fifth and sixth pedal nerves. Close to its origin on either side the 

 parapedal commissure gives off a slender nerve. That of the right side curves 

 immediately to the aorta, and bifurcates at a, Plate 5, fig. 2, into an anterior 

 and a posterior branch of equal size. The anterior branch courses forward 

 along the ventral wall of the aorta which it supplies with fine nerves. The 

 posterior branch passes backward along the aorta and soon divides into two 

 nerves. One of these, d, diverges from the aorta and passes upward and back- 

 ward into the connective-tissue diaphragm in front of the stomach. The other 

 branch, e, continues along the right ventral border of the aorta upward and 

 backward toward the heart. It sends no branches to the aorta, but gives off a 

 nerve, /. to the diaphragm, and then forms an anastomosis with a branch of a 

 pedal nerve, 9a, the united nerve thus formed, h, passing dorsally in the dia- 

 phragm to the superficial peritoneum in which it ramifies. 



Upon the left side the homologous nerve, b, from the parapedal commissure 

 loops backward in the connective-tissue bordering the left margin of the aorta 

 and follows its left pedal branch to the foot and body-wall, ramifying in the peri- 



