(Spitzer, 1937) and only traces were reported in 

 Alya. In both organisms amino acids in the ex- 

 creted material amounted to 18 percent in Mya 

 and from 17 to 35 percent in Mytilux. 



Two different methods were used in the studies 

 of nitrogen secretion. The nephridia with adja- 

 cent tissues were ground with sand and extracted 

 with distilled water, and the extracts analyzed; 

 alternately, the entire animals were placed in 

 small quantities of distilled water, kept at 16° to 

 18° C. for a period of time varying from 24 to 

 72 hours, and the metabolites accumulated in 

 the water were analyzed. It appears from the 

 experiments in which both fresh-water and marine 

 bivalves were used that uric acid is not present in 

 the nephridia. of Anodonta, Unio, Mytilus, and Mya 

 (Spitzer, 1937; Przylecki, 1922a). Przylecki 

 found that in Anodonta up to 60 percent of the 

 total excreted nitrogen is represented by ammonia, 

 and that the amount of ammonia excretion may be 

 greatly increased by placing the animals in acidi- 

 fied water for a short time. He also found that 

 ui'ease, the enzyme which converts m'ca into 



ammonia, is present in abundance in Mytilus edvlis 

 and Helix (Przylecki, 1922b). In a study of the 

 urinary functions in bivalves Letellier (1887, 

 1SS9) arrived at the conclusion that in the 

 nephridia of Mytilus, Anodonta, and Cardium 

 urea takes the place of the uric acid; and Marchal 

 (1889) was not able to find uric acid in the excre- 

 tion of 50 mussels he tested. Spitzer (1937) 

 demonstrated, however, the presence of uric acid 

 in the middle intestine of Unio and Mytilus. 

 Delaunay (1931) found traces of uric acid in Mya 

 arenaria and small quantities of it in the products 

 of nitrogen excretion in the Portuguese oyster 

 Crassostrea (Grypfuiea) angulata. 



Among the identified nitrogenous metabolites 

 excreted by Mya arenaria and Mytilus edulis 

 the largest proportion belongs to the amino 

 acids. Next in importance is ammonia, followed 

 by purine, urea, and uric acid. The numerical 

 values expressed in grams of nitrogen of a given 

 compound per total nitrogen in excreta are given 

 in table 32. 



Microns 



50 



Figure 252. — Columnar cells of the lining of the kidney reservoir of C. virginica. Basal membrane with a well-developed 

 layer of circular muscles. Oil immersion. Kahle, hematoxvlin-eosin. 



THE EXCRETORY SYSTEM 



277 



