FRESHWATER MUSSELS 61 



reference has already been made. It is really a pair of thin folds 

 of the dorsal surface hanging down to right and left, and through 

 it can be seen many of the other organs. The ventral edges of 

 the mantle are thickened, and opposite the posterior _ adductor 

 muscle they are united for a short distance. Below this region 

 of fusion each edge is pigmented, and bends away from its fellow 

 twice in quick succession. Thus at the posterior end of the 

 animal two slits are left between the right and left mantle-flaps. 

 The lower slit is provided with a fringe of short tentacles, and 

 through it during life a stream of water is constantly wafted 

 in within the space between the mantle-flaps. After circulating 

 through this space along a definite course the water passes out 

 through the dorsal slit. The two slits are respectively known 

 as the inhalant and exhalant siphons. The tentacles border- 

 ing the inhalant siphon test the quality of the entering water. 

 It will thus be seen that all supplies of food and of oxygen enter 

 the shell at the hinder end ; as indeed is inevitable, seeing that 

 the front end is deeply plunged in the mud. These water currents 

 can be made evident in the aquarium specimen by placing some 

 carmine or other coloured but innocuous liquid near the in- 

 halant siphon with the aid of a pipette. Care must be taken 

 not to shake the aquarium, and still more not to touch the animal 

 itself, as the valves will be shut and all action cease for some 

 time. On turning the mantle-fold of one side back the gills, 

 " foot " and labial palps are exposed. The gills are brown in 

 colour, two on each side of the body, and marked with fine 

 horizontal and vertical lines. Each is composed of two sheets 

 which enclose a long narrow cavity between them. In section 

 the cavity is V-shaped, open dorsally but closed ventrally. The 

 two sheets are, however, united to one another at frequent inter- 

 vals, so that the enclosed space is divided into a number of com- 

 partments. In development each gill-plate is made up of a 

 large number of separate V-shaped filaments, which produce 

 the finer vertical lines ; these filaments are united to one another 

 by horizontal lines of fusion causing the horizontal stripes ; 

 and at intervals the two limbs of the same V are united by 

 transverse vertical fusions causing the more pronounced vertical 

 stripes. Between these several fusions there are left spaces 



