282 



SWAN MUSSELS. PHYLUM MOLLUSCA 



undergoes a change into the adult form. Eventually the skin 

 enclosing the young mussel withers and it drops off to lead an 



m^ //b ^Tiy 



Fig. 209.^A glochidium larva in ventral view. — From Latter. 



b , Bysaus (cut short) ; d. future mouth ; m., adductor muscle ; s., sensory cells ; t., main teeth and denticles 



on ventral edge of each valve. 



hysszis 



shell teetTi 



steeple -sJtaped 

 sensory cells 



Fig. 210. — A glochidium larva, as cast out from the parent, viewed from 



behind. — From Latter. 



independent life. By means of this larva, the slow-moving 

 mussel is dispersed into fresh feeding grounds by the fish, without 

 the risk, which would be considerable if so small a larva were 

 free-swimming, of being carried downstream to the sea. We 



