27 
hurried along the marginal grooves of the branchial plates. These 
are formed by the extremities of each bar in close apposition, which 
terminate in two finger-like processes. The nervous system is more 
easy to make out. The labial ganglions may be distinguished in the 
fresh_mussel by their yellow colour, at the base of the labial pro- 
cesses lying on the tendons of the anterior pedal muscles. They 
are connected by a transverse chord passing above or in front of the 
mouth. From each ganglion two principal nerves are given off; 
one passes downwards and forwards to the anterior adductor, send- 
ing branches to the mouth and tentacles; the other backwards along 
the base of the foot to the posterior adductor, where it joins its 
fellow by means of a bilobed ganglion, which distributes nerves to 
the retractors, the base of the foot and the acoustic sac. The 
branchial ganglions send off branches, which diverge and supply the 
base of their respective gills; then each gives off a large nerve which 
passes over the adductor muscle to the hinder part of the pallial 
lobe, along which it curves, and is continued forwards near the 
border of the mouth until it meets the nerve which passes forward 
to the anterior adductor from the labial ganglion. These cireum- 
pallial nerves give off branches to the tentacles and the border of 
the mantle, forming a plexus. The nerves and ganglions are soft 
and pellucid, as common to the aquatic invertebrata. In the 
Lamellibranchiate Mollusca, which are acephalous, the nervous 
system only gives to the animal general sensibility to impressions. 
- Whether the labial ganglia which supply the mouth and tentacles 
enable it to select food is questionable. I rather think they do: 
still, during my search for diatoms, by boiling about a dozen 
mussels in nitric acid and carefully washing the deposit, I was only 
so far successful as to find about half-a-dozen, one of which (Trice- 
ratium favus) I haye mounted. The remaining portion of the 
deposit, with the exception of two or three Coscinodisci, consisted 
entirely of sand. Perhaps it will be safest to say that they take 
whatever comes in their way, and make the best use they can of it. 
Mr. Johnson illustrated his paper by means of numerous dia- 
grams, mounted specimens, and beautiful dissections. 
In the course of the discussion which followed the paper, Mr. 
Perry asked whether in the study of Mollusca there was any plan 
by which the parts to be dissected might be hardened? His expe- 
rience had taught him, that unless the object was hardened it was 
next to impossible to separate the tissues. 
Mr. Jounson said he had tried chromic acid, but possibly he 
applied it too strong, for it dried up the parts and rendered them 
brittle. The specimens before him were prepared in methylated 
spirits of wine, and they were tolerably firm. Perhaps the chromic 
acid might do if it was more diluted, and it was probable that spirits 
of wine alone would be effectual. 
