PIGMENT CELLS. 
327 
The Unio peaii-fi.slieries are still somewhat desultorily carried on 
ill various parts of the British Isles, North America, Germany and a 
few other countries, but were formerly of much greater importance 
and extent. 
Even at the present day the fisheries still existent in various Irish 
and Scotch rivers occasionally yield valuable pearls, but during the 
eighteenth century the Tay fishery was very productive, the value of 
the pearls found therein from 1761 — 1764 being about £10,000. 
The Welsh pearls were also noted for their lustrous beauty, the 
Conway being said to have furnished a pearl of especial purity which 
Sir Richard AVynne presented to Queen Catharine, and which, 
according to Reeve, is now set in the crown of Queen Victoria, 
although the Keeper of the Regalia .states there is no mention of 
such a gem in the description of the Crown jewels. 
The Pigment Cells {pigmenfum, paint or colour) are distributed 
throughout the body, but are perhaps most plentiful in the connective 
tissue subjacent to the external epithelium, and in the testaceous 
species are chiefly congTegated at the mantle-margin, passing in- 
sensibly into calcic and mucous cells, the three mutually intergradiiig 
as is shown in Arion rufiis, Limax flavus and other .species, in which 
pigment is associated with and stains the dermal mucus and assists 
to give the body its distinctive tint ; similar effects are exhibited 
in Agriolimax agrestis, in which the functional relationship of the 
mucous and lime cells is exemplified by the limey mucus they so 
plentifully exude. 
This expulsion of colouring and other matters from the body may 
be regarded as a true excretory function, quite analogous to that seen 
in the periodic moulting of the feathers in birds and the .shedding of 
hair in animals, as Uric acid and other colouring matters are waste 
products of the organism, which have probably been seized upon by 
the phagocytic cells and carried to the external surface of the body ; 
the exhaustion of the supply of colouring matters at the surface by its 
long continued or too copious exudation with the slime, when the 
animal has been closely confined for a length of time or greatly 
irritated, probably accounts for the instability of colouring in adult 
slugs. Under such circumstances Limax fldvm is said to pass rapidly 
from light yellow to dull olive-gTeen, and Avion atev has been recently 
observed by Air. Gain to pass in a very short space of time from 
orange-yellow, through chocolate-brown to grey, tinged and spotted 
