320 MANUAL OF ELEMENTARY ZOOLOGY 



limb has spongy walls lined with a dark, glandular epithe- 

 lium. It opens into the front end of the peri- 

 Excretory cardium by a crescentic renopericardial opening 

 Gonads. iii the floor of the latter. The upper limb 



has thin walls. It opens on the side of the body 

 into the adjoining inner epibranchial chamber, shortly 

 before the point at which the inner lamella of the inner gill 



The structure si A nod out a. — After Rankin. 



•-., Anterior adductor ; c.ft.g., cerebral or cerehropleur.il ganglia; St., 

 stomach; 7'., ventricle, with an auricle opening into it: k., kidney, 

 above which is the posterior retractor of the foot ; r. , rectum ending 

 above posterior adductor ; z'.g'., visceral ganglia with connectives (in 

 black) from cerebropleurals ; £-., gut coiling in foot ; /.c-. pedal ganglia 

 in foot, where also are seen branches of the anterior aorta and the~ re- 

 productive organs; /./..labial palps behind month. At the posterior 

 end the exhalant (upper) and inhalant (lower) siphons are seen. 



becomes free. Thus the two ends of the kidney tube cross, 

 the lower opening upwards and the upper downwards. It 

 should be noted that the swan mussel, like the earthworm, 

 the tadpole, and probably the crayfish, possesses excretory 

 organs in the form of coiled tubes which open at one end 

 into the ccelom and at the other communicate with the 



