412 



MOLLUSCS. 



nection with the Unionidce 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 (Urdo r.wrgarUifer), SHOWING PEARL WITHIN THE SHELL, AND DETACHED PEABLS (1-8). 



in 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 D. polymorpha w^as 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 Tellinidce, Scrobiculariidce, Donacidce, 

 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, 



