CH. VI] THE FRESH- WATER MUSSEL 189 



sharp triangular tooth bearing numerous small secondary 

 teeth upon its outer surface. The whole surface of the 

 valves is pierced with numerous fine holes. An embryonal 

 mantle lines the shell and bears on each side four steeple- 

 shaped sense organs. A simple powerful adductor muscle 

 is present, round which is coiled a sticky thread, the 

 byssus, whose free end projects from a median papilla 

 for a considerable distance. 



Zyssus 



shell teeth 



steeple - 

 sensory ceUs 



Fig. 34. Glochidium larva, as cast out by parent, viewed from behind. 



Hitherto the developing larvaB have been both nour- 

 ished and held securely within the brood-chamber, af- 

 forded by the outer gill-plate, by a nutritive mucus secreted 

 by the epithelium of the junctions between the outer and 



