There are also a clean race of frogs and tortoises, and a few mussels in it; muskrats and minks leave their traces about it, and occasionally a travelling mud-turtle visits it.
Over 40 species of freshwater mussels (Unionidae) have been observed in the Rumanian rivers.
In the statistics for England and Wales mussels are not separately distinguished.
The genital papilla of the female acquires a great development during the breeding season and becomes produced into a tube nearly as long as the fish itself; this acts as an ovipositor by means of which the comparatively few and large eggs (3 millimetres in diameter) are introduced through the gaping valves between the branchiae of pond mussels (Unio and Anodonta), where, after being inseminated, they undergo their development, the fry leaving their host about a month later.
Such movement is more frequent in young mussels than in the full-grown.
Experiments made by removing mussels from salt water to brackish, and finally to quite fresh water show that it is even more tolerant of fresh water than the oyster; of thirty mussels so transferred all were alive after fifteen days.
They are somewhat remote from the sea mussels in structure, and have not even a common economic importance.
Its original name is said to have been Eskmouth, its present one being derived from a bed of mussels at the mouth of the river.
Oysters and mussels are obtained on the East Scheldt, and anchovies at Bergen-op-Zoom; while salmon, perch and pike are caught in the Maas, the Lek and the New Merwede.
The quantity of mussels landed on Scottish coasts has decreased in recent years owing to the decline in the line fisheries.