IX MOLLUSCA 345 



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



H r^ 



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 ; 6r, rudiments of gills ; eg, cerebral ganglion ; c.^, 

 cerebral pit ; /, foot ; i/, heart ; hep, liver ; fc, 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 



