V.] THE FRESH-WATER MUSSEL. 341 



cavity. Three sets of bristle-like appendages are 

 developed in connection therewith; an upper 

 one passing obliquely downwards immediately 

 beneath the adductor muscle, and two shorter 

 ones passing upwards, immediately above the 

 shell-teeth. 



e. The byssus; an elongated, transparent, much- 

 tangled filament, projecting beyond the free edges 

 of the valves. 



d. Examine a similar larva from the side. The valves 

 are quite transparent, and there can be seen through 

 them 



a. the adductor muscle; appearing as a median dark 



mass. 

 (3. The visceral mass; a somewhat less conspicuous 



mass lying in one of the top corners, altogether 



to one side, 

 y. The byssus organ; a conspicuous transparent 



tube, arising from /?. and coiled upon itself. It is 



unpaired, being present on one side of the body 



only. 

 8. Note the shape of the shelly valve; it tapers off 



below to form the shell-tooth. Focus to the 



level of this, and examine its serrations. The 



valve is seen to be dotted all over by minute 



perforations pore-canals. 



