CHAPTER XII 

 THE EXCRETORY SYSTEM 



Page 



Anatomy of the excretory system _ 271 



Histology __ _ _ _ 273 



Physiology 274 



The waste products 276 



Osmoregulation _ 278 



Bibliography _. ___ __ 279 



End products of bivalve catabolism are excreted 

 by the nephridia, pericardial glands, wandering 

 phagocytes, and the mantle epithelium. The 

 urinary function, which is the principal activity 

 of the excretorjr system, is performed by the paired 

 nephridia situated on either side of the visceral 

 mass near the heart. The pericardial glands, as 

 the name indicates, are located on the wall of the 

 pericardium but in the oysters and some other 

 species are represented by special cells on the outer 

 wall of the auricles. The wandering phagocytes 

 may be found throughout the tissues of the visceral 

 mass and the gills. They accumulate on the sur- 

 face of the body by diapedesis and are discarded. 

 Mucus or goblet cells of the surface epithelium 

 are, in addition to their primary- function of 

 secreting mucus, involved in excretion and cany 

 within their bodies various granules which contain 

 pigments and hea\y metals (see: ch. XVII). 



The principal part of the excretory- system of 

 bivalves is similar to that of annelids. It consists 

 of a pair of tubular nephridia (fig. 244, 1. neph., 

 r. neph.) which retain direct communication with 

 the pericardium and sex glands on one side and 

 open to the outside through a short passage. The 

 coelom of bivalves is reduced to two separate 

 spaces, the pericardial cavity and the inside of tlie 

 gonad tubules. The ducts leading from the 

 nephridia and gonads open to the outside either 

 independently or through a common reno-gonadial 

 vestibule. 



Comprehensive reviews of the anatomy and 

 histology of the excretory system in mollusks may 

 be found in the papers of Odhner (1912), Strohl 

 (1924), Haas (1935), Spitzer (1937), and Franc 

 (1960). 



FISHERY bulletin: VOLUME 64, CHAPTER XII 



ANATOMY OF THE EXCRETORY 

 SYSTEM 



The excretory organ of the oyster occupies an 

 indistinctly outlined triangular area on either side 

 of the visceral mass. On the surface its location 

 is marked by light brownish pigmentation. The 

 organ consists of a central part which lies between 

 the pericardium and the adductor muscle and two 

 branches or limbs which extend along both sides of 

 the body. The right limb is slightly longer 

 than its opposite member. Excretory- tissues are 

 found directly under the surface epithelium; they 

 are surrounded by branchial vessels and numerous 

 blood sinuses. 



The relation between the different parts of the 

 excretory system, the heart, and the adductor 

 muscle is shown diagrammatically in figure 244. 



Each nephridium of a bivalve is bent into a 

 U-shaped tube which forms many convoluted 

 branches penetrating the surrounding tissues. 

 The narrow central part of the excretory system 

 extends across the body from one side to the other 

 and contains a narrow and twisted inter-nephridial 

 passage (i.r.p.) which connects the two limbs. 

 Fingerman and Fairbanks (1958) think that this 

 passage "does not appear to be present in 

 Crassostrea virginica." Reconstruction of the 

 structure of this area from serially sectioned 

 material shows that there is a communication 

 between the right and left nephridia although the 

 passage is narrow and greatly twisted (fig. 245) 

 and, therefore, appears in the preparations as a 

 series of circles and irregular cylinders. Exami- 

 nation of a number of consecutive sections shows 

 an uninterrupted connection between the right 

 and left nephridia. 



The nephridium of each side begins inside the 

 pericardium by the reno-pericardial opening (fig. 

 244, r.p.o.), which leads through a ciliated funnel, 

 the nephrostome and a short reno-pericardial 

 canal (fig. 244, r.p.c, and fig. 246), to the body of 

 the kidney. Both limbs of the kidney are formed 



271 



