oF 
INTERNAL ANATOMY. B07 
paired nerve arising from the parapedal commissure described below. It sends 
a branch to the peritoneum of the diaphragm-like 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 (1886) as existing in D. 
rumphu [= 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. 2, 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, f. 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- 
