ix MOLLUSCA 345 



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



a 



D 



si 



r.f 



FIG. 275. Side views of two young specimens of Drcisseiisin /.li/nioi-/>ln/ niter tlic 

 metamorphosis has taken place. (After Meiseiiheiiner. ) 



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

 specimen in which the shell is beginning to assume its adult proportions, a, anus; <!il.<i, anterior 

 adductor muscle ; mlil.p, posterior adductor muscle ; In; rudiments of gills ; <:y, cerebral ganglion ; c.p, 

 eerebral pit; /, foot; H, lieart ; /<>/), liver; 7,-, kidney; l.p, labial palp; uf, otneyst ; per, pericardium ; 

 JLIJ, pedal ganglion ; r.f, retractor of foot ; st, stomach ; r.it, visceral ganglion. 



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



