ANODONTA. 275 



to the visceral ganglia situated immediately below the pos- 

 terior adductor muscle. There is here less concentration 

 than in the snail, the pedal and visceral loops being very 

 long and wide. 



Immediately under the pericardium lie the paired kidneys. 

 They consist of tubes bent upon themselves. Each has 

 an internal opening into the anterior end of 

 the pericardium, which passes into the lower 

 excretory part or kidney. From the posterior end of each 

 kidney a ureter passes forward between it and the peri- 

 cardium to open into the inner supra-branchial chamber, 

 and thence to the exterior. These tubes may be regarded 

 as two specialised nephridia. The walls of the pericardium 

 also have excretory cells, which are known as the pericardial 

 glands (organ of Keber). 



Anodonta is ' dioecious. The testis or ovary \sy a diffuse 



paired organ lying below the kidneys. The 



' paired genital duct (oviduct or vas deferens) 



passes up and opens just below the excretory pore on each 



side. 



The eggs are shed into the supra-branchial chamber, 

 where they are fertilised and develop into glochidia^ or 

 _ small two-shelled larval forms, which differ in 



' many respects from their parents. They leave 

 the parent by the exhalent aperture. A little dorsal to the 

 exhalent aperture, the two mantle- edges again diverge to 

 form a small slit-like aperture. This is connected by a 

 median canal above the intestine with the exhalent cham- 

 ber, and embryos have been observed escaping by it. The 

 glochidium is said to be parasitic upon certain fish, and 

 undergoes a metamorphosis into the adult. 



The general likeness of Anodonta to the snail will be 

 apparent. The piano-symmetry is, however, more perfect, 

 shown in the paired shells, kidneys, auricles, gills, &c. The 

 absence of buccal mass, odontophore and eyes, and the 

 immense development of the ctenidia (which, present in 

 most Gastropoda, are absent in the snail) are the chief points 

 of distinction. 



