ix MOLLUSCA 345 



way the external opening is formed. The greater part of the ureter 



FIG. 275. Side views of two young specimens of Dreissensia polymorpha after the 

 metamorphosis has taken place. (After Meisenheimer. ) 



A, young specimen in which the shell retains the shape it possessed in the veliger. B, older 

 specimen in which the shell is beginning to assume its adult proportions, a, anus ; add.a, anterior 

 adductor muscle ; add.p, posterior adductor muscle ; br, rudiments of gills ; c.g, cerebral ganglion ; c.p, 

 cerebral pit ; /, foot ; H, heart ; hep, liver ; k, kidney ; l.p, labial palp ; ot, otocyst ; per, pericardium ; 

 p.g, pedal ganglion ; r.f, retractor of foot ; st, stomach ; v.g, visceral ganglion. 



is therefore not ectodermal but coelomic in origin. The limb of the 



