PROCEEDINGS OE THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
13 
system of the mussel consists of three nerve masses or 
centres, or, more correctly, of three pairs of nerve masses, 
connected with one another by a system of nerve- 
cords. The first of these centres of “ ganglia ” is situated 
near the mouth, and is probably chiefly concerned in con¬ 
trolling the digestive organs; the second lies in the foot; 
and the third is associated with the adductor muscles. 
The only organ of sense which has been discovered is one 
of hearing;—an ear it can scarcely be called, as it is merely 
a little sack containing fluid, and calcareous particles 
termed “ otoliths.” 
The young mussels are developed from eggs, of which 
some 200,000 are computed to be produced by each 
individual in a year, though the number is some¬ 
times very much greater. The spawn is kept within 
the shell of the parent mussel until the eggs are batched. 
The young or “ larval” mussel is so unlike a mature speci¬ 
men, that, did we notknow somethingof its history, it would 
be impossible to recognise it as the same creature. The shell 
is thin and globular, each valve being triangular, and 
terminating in a sharp hook; the mouth is very wide; the 
foot small; and the gills scarcely developed. The first few 
days of its free existence it spends in a very lively manner, 
swimming about through the water by rapidly opening and 
closing the valves of its shell, or attaching itself to some 
floating object, sometimes even fixing itself to the tail of an 
unlucky fish by means of the sharp hooks with which its 
shell is provided. This free-swimming condition in which 
the mussel spends the early part of its existence is an 
admirable provision for the dissemination of the young 
molluscs, which would otherwise be deposited in such 
large numbers in one place that they would not be able to 
obtain sufficient food; and in the retention of the spawn 
within the shell of the parent we see a provision for 
protecting it from being preyed upon by fish. At length 
the young mussel enters on a more sober and settled exist¬ 
ence, spending the rest of its days among the stones or mud 
of the bed of the river. It generally lies with the rounded 
end of the shell imbedded in the sand or mud, and the 
valves partly open,—the opening facing down the stream, 
so that the back of the shell acts as a breakwater to divide 
the current. The cilia of the gills create a constant flow 
of water through the shell, which not only renews the 
blood, but brings a supply of food to the mouth. The food 
consists of minute organisms, such as Infusoria, Diatoms, 
&c., besides particles of vegetable matter, as more solid 
material could not be assimilated without a masticatory 
apparatus such as we find in the higher molluscs. The 
mussel is very susceptible to external influences, instantly 
closing its shell when disturbed; and, although no organ of 
sight has yet been discovered, it is found to be considerably 
affected by light, keeping its shell open on a bright sunny 
day, but closing it when the sky is overcast. At times it 
manages to crawl to a considerable distance by means of 
its foot, and is observed to be most active in spring, just 
after the spawning season. 
As regards habitat, the pearl mussel is generally found 
in swift-flowing rivers, and in the streams and lakes 
of mountainous districts. This explains its distribu¬ 
tion in Britain, where it occurs in Devon, Cornwall, 
North and South Wales, the mountainous districts of 
Cumberland, the rivers flowing from the Highlands 
of Scotland, and those of the north and south of Ireland. 
In Scotland I have collected specimens from the Tay, the 
Tummel, the Garry, the Earn, the Forth, the Teith, the 
Allan, the Doon, and Loch Tummel. On the Continent it 
ranges through all the hilly regions, from Lapland to the 
Alps and Pyrenees, which form its southern limit. It is 
particularly abundant in Norway and Sweden, and Prof. 
Forbes believes the Scandinavian Peninsula to have been 
the centre from which it has spread through Europe. The 
scientific name of the present species, Unto margaritiferus, 
was given to it by Linne, in reference to its pearl-bearing 
propensity. The family to which it belongs is the Unioni- 
dae, the family of the fresh-water mussels. The chief home 
of the family is in the rivers of North America,—the United 
States, alone yieldingover 100 species,—whilein Europe the 
number both of species and of individuals is comparatively 
small. The family embraces in all 420 species, and its 
range is very wide, extending over North and South 
America, Europe, Asia, Africa, and Australia. In a fossil 
state it is represented in the Wealden of Europe and 
India. Besides Unio margaritiferus, three species belong¬ 
ing to the same family occur in Britain, namely, U. 
tumidus, Philippsson, and U, pictorum Linne, which are 
confined to the streams and canals of Southern and Central 
England; and Anodonta cygnea Linnb, which extends into 
Scotland. 
I am not aware that the pearl mussel has ever been 
used as an article of food in this country, but in the 
South of France it is eaten in considerable quantities. 
Regarding its qualities as an article of diet, I can say 
nothing from personal experience ! 
The manner in which pearls are formed long re¬ 
mained a mystery, and many absurd notions prevailed 
regarding it. An article in the Quarterly Review for 
1815, describing one of these popular fallacies, says: 
—“The Arabs, with whom the pearl was an article 
of great commerce, entertained a notion, which they 
had from the Brahmins, that when it rained the 
