v.] THE FRESH-WATER MUSSEL. 34I 



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, 



€. 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. 



^. The visceral mass ; a somewhat less conspicuous 

 mass lying in one of the top corners, altogether 

 to one side. 



y. The byssus orga?i; a conspicuous transparent 

 tube, arising from /3. 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-catials. 



