XL] THE FRESH-WATER MUSSEL. 113 



lateral 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 ap- 

 proach, 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. What appears to be 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. 



After a time the larval Anodontcz leave the body of the 

 parent, and attach themselves to floating bodies very com- 

 monly to the tails 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 undergo a metamorphosis; the gills are developed, 

 the foot grows, the auditory vesicles become conspicuous in 

 it, and the young Anodonta at length drops off and falls into 

 its ordinary habitation in the mud. 



LABORATORY WORK. 



i. In the natural state of the animal only the shell or 

 exoskeleton is visible, or this may be slightly open, 

 and then the edge of the membrane lining it (the 

 mantle] may be visible. Raise one valve of the shell, 

 by separating the mantle from it with the handle of a 

 scalpel, and then cutting through two strong bodies 



3 



