EXCRETOR Y S YSTEM. 



409 



Excretory system. The excretory system is difficult to dissect and 

 to explain. On each side of the anus there is a little papilla, through 

 which uric acid and other waste products ooze out into the mantle 

 cavity, and so into the water. A bristle inserted into either of these 

 two papillae leads into a large sac the nephridial sac. But the two 

 sacs are united by two bridges, and they give off an unpaired dorsal 

 elongation, which extends as far back as the reproductive organs. 



The dorsal wall of each nephridial sac becomes intimately associated 

 with the branchial veins, and follows their outlines faithfully. It is 

 likely that waste material passes from the blood through the spongy 

 appendices into the nephridial sacs. 



FIG. 219. Diagram of circulatory and excretory systems 

 in a Decapod-like Septa. After Pelseneer. 



i, Gill ; 2, renal sac ; 3, afferent branchial vessel ; 4, branchial heart ; 

 5, abdominal vein ; 6, heart ; 7, yiscero-pericardial sac (body 

 cavity); 8, genital organ; 9, posterior aorta; 10, "auricle" ; n, 

 glandular appendix of branchial heart ; 12, renal appendices of 

 branchial vein; 13, external aperture of kidney; 14, vena cava; 

 15, anterior aorta ; 16, bifurcation of vena cava ; 17, reno-pericardial 

 aperture. 



Into the terminal portion of each nephridial sac, a little below its 

 aperture at the urinary papilla, there opens by a ciliated funnel another 

 sac, which is virtually the body cavity. It surrounds the heart and 

 other organs, and is often called the viscero-pericardial cavity. 

 Through the kidneys or nephridial sacs it is in communication with the 

 exterior. Associated with the branchial hearts there are numerous 

 diminutive cells which contain ammoniacal salts, phosphates, pigment, 

 etc. ; these waste products are probably passed into the blood and got 

 rid oft" by the kidneys, just as, in a Vertebrate, the urea formed in the 

 liver passes by the blood to the kidneys. In Invertebrates there 

 is often this co-operation between "closed kidneys" and "open 

 kidneys. " 



