V.] THE FRESH-WATER MUSSEL. 313 



scribed for the Snail — the original bilateral symmetry is 

 however never disturbed. 



The embryos when hatched, are so wholly unlike the 

 parent Anodonta, that they were formerly thought to be 

 parasites, and received the name of Glochidium. Each is 

 provided with a bivalve shell, and each valve has the form 

 of an equilateral triangle united by its base with its fellow, 

 by means of an elastic hinge, which tends to keep the two 

 wide open. The apex of the triangle is sharply incurved, 

 and is produced into a strong serrated tooth, so that when 

 the valves approach, these teeth are directed towards one 

 another. The mantle is very thin, and the inner surface of 

 each of its lobes presents three papillae, terminated by fine 

 pencils of hair-like filaments. The oral aperture is wide, 

 and its margins are richly ciliated. There is a single ad- 

 ductor muscle and a rudimentary foot, from which one or 

 two long structureless filaments, representing the byssus of 

 the sea-mussel, proceed. These byssal filaments become 

 entangled with one another and tend to keep the ' Glochi- 

 dia ' in their places. 



The gill-laminae of Anodonta will accommodate some 

 three millions or more of these Glochidia. If the animal is 

 living in company with fish it will eject them, whereupon 

 they attach themselves to floating bodies — very commonly 

 to the tails, fins or gills, of fishes— by digging the incurved 

 points of their valves into the integument in the latter case, 

 and holding on by them as if they were pincers. In this situa- 

 tion they become encysted in an epidermal overgrowth of 

 the host's body, within which they undergo a final metamor- 

 phosis. 



The alimentary canal and foot become more marked with 

 the increased development of the whole body; the gills 

 appear, in the form of filamentous outgrowths of the body- 



