412 
MOLLUSCS. 
nection with the Unionidos is the parasitic life of the early stages. The eggs, 
after being hatched between the gills of the parent, and having undergone the 
first stages of development, subsequently develop into minute bivalves, each valve 
having a hook-like process on the front edge, and are expelled from the brood 
pouches. They at once attach themselves to some extraneous object, by means of 
a byssal thread, and at this stage are known as Glochidia. They attach them¬ 
selves, as soon as possible, to the gills and other parts of fishes, by means of the 
valve-hooks, and there complete their metamorphosis, finally quitting their host, 
sinking to the bottom, and assuming the parent form. Four species are found 
pearl-mussels (Unio r.iargariti/er), showing pearl within the shell, and detached pearls ( 1 - 8 ). 
m Britain, one of which ( Unio margaritifer ) is famous for the pearls it 
produces. Some of the rivers of Scotland, and the Conway in North Wales, 
have always had a great reputation for their pearl-fisheries. Although not equal 
in lustre to the Oriental jewel many of these river pearls are beautiful. Unlike 
Unio, the Dreissensiidce , as represented by Dreissensia, have the mantle-margins 
united ventrally, with an anterior opening for the slender foot and byssus, and 
prolonged posteriorly into two siphons. The shell is shaped like the common 
marine mussel. The European J). polymorpha was first noticed in England about 
seventy years ago, and is supposed to have been introduced attached to 
Russian timber. 
Suborder Tellinacea. 
This group includes the families Tellinidoe, Scrobiculariidce, Donacidcc, 
Mactridce, Mesodesmatidce, and Cardiliidce, the first of which is the most ex¬ 
tensive, and contains the most beautiful forms. Here the animal is remarkable 
for the great length of the slender separated siphons, the fringed mantle-margins, 
