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W. J. Dakin 
the palliai nerves (fig. 1 pali. n). It innervates the osphradium directly 
on the left side (fig. 1 os''') but as described above enters the osphradio 
branchial nerve on the right. The nerves leaving the right side of the 
visceral ganghon are however all crowded, and this probably accounts for 
the fusion of the posterior nerve branch, which remains more or less 
independent on the left side. The remaining nerves all lie deeper in the 
connective tissue and close to the muscle. They arise from the dorsal 
surface of the lateral lobes (fig. 2). The PaUial Nerves (pali. n. and p. 
pali.) arise in two groups, the radial paUial nerves and the posterior 
palliai nerves (fig. 1, 2 p.pall). On the left side the radial paUial nerves 
are almost equally spaced, lie in one plane, and radiate fan-like from 
the lateral lobe (fig. 6). On the right side these nerves are not se- 
parated so distinctly, and they leave the ganghon in a broad trunk, re- 
maining in close contact until the mantle is reached. Underlying the 
radial and posterior palhal nerves are the muscle nerves. They arise 
from the dorsal surface of the lateral lobes nearer the median line and 
remain deeply situated, innervating the adductor muscle. No nerves 
arise from the dorso-central lobes. 
The Innervation of the osphradium in Pecten has been described 
above, but a few summarising remarks may be made here. Owing to 
the distance of the osphradium from the ganglion and the extent of the 
organ, the nerves are particularly distinct, and at the same time quite 
peculiar in their origin and distribution. The osphradial nerve fibres 
(fig. 7 OS. n) leave the visceral ganghon by three nerves, the branchial, 
the osphradio -branchi al, and the posterior nerve (fig. 1 os.p). The 
branchial nerve however receives some fibres direct from the cerebro- 
visceral connective. It is natural to expect that the branchial nerve 
contains fibres from the cerebral ganglion, since as will be shown later, 
the same applies to the paUial and probably muscle nerves. It is 
impossible to say whether these fibres take any part in the Inner- 
vation of the osphradium, especially since the branchial nerve receives 
fibres from the osphradio-branchial just before innervating this sense 
organ. In any case the bulk of the fibres are to be traced diiectly to the 
visceral ganglion. This description of the visceral ganglion applies in the 
main to Pecten maximus, but P. jacobaeus and P. opercularis agree in 
almost every particular, except that the lateral lobes are most unlike 
in size in P. jacobaeus and most alike in P. opercularis (of those three 
Speeles). 
