AY4 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCLETY. 
pigment granules are then seen to be present to a small 
extent in the cells, they are not frequent enough to give 
any conspicuous colour, and the organs differ greatly 
from the very obvious pigmented osphradia of many 
molluscs. Two small gangha, the osphradial gangha, 
have already been described in the section on the nervous 
system. They lie in close proximity to the visceral 
ganglion, and nerves pass upwards from the ganglion cells 
forming these two spherical ganglia to the surface 
epithelium, almost directly above, where there is a small 
prominent area of elongated epithelium forming the 
osphradium. If a piece of the adductor muscle is 
removed so that it bears on its surface the visceral ganglia 
and overlying connective tissue and epithelium, and 
sections are cut at right angles to the surface, the 
osphradial ganglia and their connections with the 
osphradia can be easily followed. The epithelial cells of 
the osphradium increase in height until they are about 
three times that of the adjoining ordinary epithelium. 
The cells forming the organ bear no cilia, though these 
oceur on the ordinary adjacent epithelial cells, but there 
is a prominent cuticle present. The nucleus is large and 
almost round, and small pigment granules occur (though 
in small numbers) ii the cytoplasm. Underlying the 
osphradia are nerve cells connected with the innervating 
nerves from the ganglia, and from these, numerous fibrils 
arise and pass through the connective tissue basal 
membrane, until, branching still further, they pass 
between the supporting epithelial cells to the surface 
(text-fig. 3). 
THE ABDOMINAL SENSE OrGAN was described first by 
Thiele in a species of Arca in 1887 (39), and List in 
1902 (6) described the structure in detail in the Mytilidae. 
In Pecten maximus it is well developed but is unpaired. 
