CLASSIFICATION 27 



are enclosed between the two, right and left, lobes of 

 the mantle, which in their turn are covered by the 

 two shelly plates, or valves. In a few instances the 

 mantle is extended and reflected over the valves, and 

 partly (Galeommidae) or completely (Ephippodonta, 

 etc.) envelops them. 



The mantle lobes are attached along the back and 

 extend out to the margins of the valves. In the 

 more primitive forms the mantle margins are quite 

 simple, and open from the front round the ventral 

 edge to the back ; but many different modifications 

 take place in the higher forms. Instead of the 

 margins being simple, there may be folds, thicken- 

 ings, protuberances, tentacles (Plate XV., Fig. 5), 

 various glands, and even eyes (Plate XV., Fig. 8). 

 Nor are the margins always free ; indeed, in extreme 

 cases they are united nearly the whole way round. 

 There are various stages (Plate IV., Fig. 4, A-F) : 

 reckoning the open one as the first (A), then in the 

 second (B), instead of the water being admitted all 

 the way round, the edges of the mantle are kept 

 closely applied to each other except where the foot 

 protrudes and at two points at the hinder end, one 

 of which serves to admit the fresh water to the gills, 

 and is called the " inhalent aperture," whilst the 

 other allows the fouled water to escape, and is 

 termed the " exhalent aperture." In the next stage 

 (C), the edges of the mantle are united permanently 

 at the point between these two openings ; and in the 



