CH. Vl] THE FRESH-WATER MUSSEL 185 



The ovaries or testes, as the case may be, occupy 

 the greater part of the visceral portion of the foot. Fre- 

 quently the ovaries have an orange tinge while the testes 

 are whitish : microscopic examination of their contents is, 

 however, the only trustworthy means of determining the 

 sex. The genital ducts pass forward, parallel with and 

 ventral to the ureters, to the genital apertures which lie, 

 right and left, immediately ventral to the nephridial 

 opening. 



At the spawning season, which takes place in May 

 and June with Unio, but in June, July and August 

 with Anodonta, the animals move towards the shallower, 

 and therefore warmer, water. There is no actual union 

 of the sexes: the spermatozoa of the males are simply 

 discharged into the water and are carried by the ciliary 

 currents into the inhalant siphon of the female. If a 

 female be taken from the shell at this season the eggs 

 may be seen through the transparent wall of the oviduct 

 passing singly, but in a steady stream, to the genital 

 aperture. Their motion is due partly to "labour con- 

 tractions" of the intrinsic muscles of the foot and partly 

 to the ciliated lining of the oviduct itself. One by one 

 the eggs issue from the genital aperture, whence they are 

 conveyed backwards by the abundant cilia which clothe the 

 external surface of the nephridium. Along the middle 

 line of this surface there is a belt of especially long cilia 

 which appear to be devoted to the transit of the eggs; 

 those dorsal and ventral to the belt work obliquely so as 

 to keep the eggs in contact with it. It is probable that 



the free dorsal border of the inner lamella of the inner 



m 



\ B R A * 

 or THE 



UNIVERSITY 



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