The visceral ganglion of Pecten, etc. 
27 
Sub-Order Myacea. Fam. Myidae. Mya truncata; Fam. Sole- 
nidae. Ensis ensis. 
Sub-Order Adesmacea. Fam. Pholadidae. Pholas dadylus. 
Arca is particularly interesting because of the discovery of the 
osphradium in this animai by Spengel in 1881. The species examined 
was the same as that investigated by Spengel namely Arca noae. Both 
the osphradium and the abdominal sense organs are well developed in 
this lamellibranch and the former are visible to the naked eye by reason 
of their pigment. They are represented by two pigmented narrow bands, 
almost lines, separated by a short distance in the median line, and their 
inner ends terminate in fan or clubshaped expansions. Spengel states 
that each organ "setzt sich wie das Geruchsorgan der Gastropoden aus 
zwei Bestandteilen zusammen, nämlich einem hohen Zylinderepithel, 
dessen Begrenzung durch die dunkle Pigmentierung sehr deutlich be- 
zeichnet ist, und aus einem unter diesem gelegenen Ganghon, das etwa 
die gleiche Ausdehnung wie das Zylinder epithel hat". This latter part 
is particularly interesting for it refers to the ganglionated branchial nerve. 
In fact Spengel goes on to declare that Duvernoy and other investi- 
gators of the nervous system had taken this ganglion for the branchial 
nerve, which was impossible because the molluscan ganglia had a cortex 
of ganglion cells and the nerves possessed no such thing. For this reason 
alone, Spengel considers the branchial nerve as a ganglion. As a matter 
of fact the branchial nerve in all the species examined has a cortex of 
ganglion cells. The visceral ganglia (fig. 16) a pair of simple lobes, remain 
a little distance apart, united by a broad transverse commissure. Ante- 
riorly two nerves, the cerebro -visceral connectives (c. v. con) leave the 
ganglia and since these nerves lie near the surface they can be followed 
without dissection. Several smaller nerves leave the anterior surface 
of the ganglion. From the posterior ends of the ganglia arise two nerves 
one of which, the branchial (fig. 16 hr.n) turns off at right angles, whilst 
the other and more posterior of the two passes backwards over the muscle 
as the posterior palliai nerve (p. pali). The osphradia lie just above 
the branchial nerves, beginning at the edge of the visceral ganglion and 
a number of osphradial nerves (fig. 16 os.n) leave the branchial nerves at 
irregulär intervals. No fibres pass directly from the cerebro-visceral 
connective to the osphradium. 
P e ctunculus gly cimeris, another representative of the 
family Arcidae has been examined. The two visceral ganglia are fused 
together, and it is impossible to trace any definite boundary between 
