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 lhe the paired Azdneys. 
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 dicecious. The ¢estis or ovary is 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 plano-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. 
Excretory. 
Reproductive. 
Development. 
