irp 



Centimeters 



0.5 



Figure 245. — Caniora lucida drawing of a left portion of tlie central part of the excretory organ of C. virginica seen on 

 cross section. The beginning of the inter-nephridial passage (i.r.p.) at the junction with the reservoir, R, is on the 

 left side; Ad.M. — adductor muscle. Bouin, hematoxylin-eosin. 



tached considerable importance to this morpho- 

 logical relationship for which he coined the term 

 "ektaxial." 



In the absence of a generally accepted ana- 

 tomical termuiology for the excretory system of 

 bivalves, considerable confusion exists in the lit- 

 erature because of the great variety of names and 

 descriptive terms aoplied to identical parts. The 

 nephridium was discovered m 1817 by Bojanus 

 (1819) who mistook it for the organ of respiration. 

 .Since then the excretory system of mollusks has 

 been known in zoological literature as the organ of 

 Bojanus. There seems to be no advantage in the 

 conthiued use of this name, because the organ is 

 frequently called kidney, nephi-idium, renoperi- 

 cardial or nephropericardial passage (Renoperi- 

 cardialgang, in German), and nephridial sac in 

 cephalopods. The dorsal limb of the nephridium 

 may be called the proximal part, inner limb, peri- 

 cardial part, nmer sac, excretional part, antero- 

 posterior hmb, and kidney sac. The ventral (or 

 distal) limb is also knowii as outer sac, efferent 

 part, and kidney passage. The reservoir is some- 

 times referred to as vesicle and bladder. The 

 reno-pericardial opening is called nephrostome, 

 nephridial funnel, and "Nierenspitze" or kidney 

 syringe (Haas, 1935). 



HISTOLOGY 



The glandular nephridial tubes of the oyster are 

 lined with a special epithelium; the cells are 



THE EXCRETORY SYSTEM 



characterized by clear cytoplasm and absence of 

 granules. Three kinds of epithelial cells are found 

 in the different parts of the excretory system. In 

 the tubules of the anterior limb the cells are of 

 medium size, cylindrical, and vacuolated (fig. 249). 

 The lining of the internephridial passage and of 

 the tubules originating from its wall is made of 

 short, almost cubical cells with inainspicuous 

 vacuoles (fig. 250). In places the epithelium 

 lining is two or three cells deep. Tall columnar 

 cells with large terminal vacuoles are found pri- 

 marily in the tubules of the posterior limb and in 

 the reservoir (fig. 251 and fig. 252). The cells 

 are about seven to nine times longer than their 

 width. The vacuoles have no granules visible 

 under the light microscope; they appear to contain 

 liquid as if they are ready to burst. Some of the 

 vacuoles seem to be empty, and many separated 

 vacuoles are found in the lumen. . 



The lining of the reservfiir consists of columnar 

 cells of different heights; some of them are twice 

 or three times taller than the others and protrude 

 into the lumen (fig. 252). The epithelium rests 

 on a basal membrane with a well-developed layer 

 of cu-cular muscles which extends to the renal duct. 

 There is no organized sphincter for the control of 

 the flow of urine. The contraction of the bladder 

 in response to the fluid flowing into it from the 

 pericardial cavity was observed by Fingerman and 

 Fairbanks (1958). 



273 



