CH. VI] THE FRESH- WATER MUSSEL 181 



The orange-coloured pigment, which is universally 

 present in the skin and particularly brilliant in that 

 of the foot, and also on the surface of the nerve 

 ganglia, is known as tetronerythrin and possesses a strong 

 affinity for oxygen. It is noteworthy that this respiratory 

 pigment is especially well developed in the foot, where, 

 as already indicated, the blood is in a stagnant con- 

 dition whenever the animal is moving. There can be 

 little doubt that the blood imprisoned in the pedal 

 lacunae during turgescence is maintained duly supplied 

 with oxygen by the united action of this pigment and 

 diffusion accelerated by the stretched condition of the 

 skin. 



Excretion 1 . The elimination of waste products is 

 brought about by the portal circulation within the 

 glandular substance of the nephridia or organs of 

 Bojanus 2 , as they are often termed. These organs lie 

 right and left ventral to the pericardium, with whose 

 cavity they communicate, thus establishing connexion 

 between it -and the outside water through the ureter. 

 Within their substance numerous nitrogenous compounds 

 allied to urea and uric acid have been identified. 



Keber's organs, a pair of rusty red glandular prolifera- 

 tions of the anterior walls of the pericardium, also perform 

 similar excretory functions. They are supplied with 



1 Griesbach, Arch. f. Naturges. XLIII. 1877; Krukenberg, Vergl. 

 physiol. Stud. i. 2; Bankin, Jena Zeitsch. xxiv. 1889-90. 



2 Louis Henri Bojanus, born July 16, 1776, at Buschweiler in Alsace ; 

 died April 2, 1827, at Darmstadt ; Professor at the University of Vilna. 

 He was especially noted for his works on comparative anatomy and 

 veterinary medicine. 



