ANODONTA — INTERNAL ORGANIZATION, ETC. 1G7 
line. The nervous system is also of a paired character, but with tlie 
different centres widely separated and developed correspondingly to 
the morphological changes in the animal. The alimentary system is 
convoluted more or less medially, the mouth being placed in the 
middle line anteriorly beneath the anterior adductor, the termination 
of the digestive system being at the 
posterior end of the body. The 
circulatory organs are dorsally 
jdaced, the ventricle in the median 
line, and enclosing the rectum, with 
a symmeti'ically placed auricle at 
each side, which receives the blood 
from the branchiie adherent to 
the same side of the body. The 
glandular system, when localized 
in definite organs, is also distinctly 
paired, the liver being formed of 
two lobes and the excretory neph- 
ridial and pericardial glands have 
their component parts placed to the 
right and left of the me<lian line. 
The muscnlar system partakes 
strongly of the same bilateral uni- 
formity, the various muscles or 
retractors arising from one side of 
the body having their counterparts 
at the other. The reproductive 
organs are also paired, genital 
glands being present at both sides of the body, and each furnished 
with a duct debouching into the cavity of the inner gill of their 
respective sides. 
All the organs of the body may be advantageously studied, as in 
the Gastropoda, in connection with the particular system to which 
they have the most intimate relations. 
The Nervous system of Anodonta is of the simplest type, in which 
well-marked ganglia exist, and is composed of three chief medullary 
masses or ganglia, which are placed at considerable distances from 
each other, but connected by long nervous cords, forming two very 
long nerve rings or nerve loops, which, however, evidently confer a 
a. auricle of the heart ; y. foot, showing 
the cut sections of intestinal tract ; i.c. inner 
ctenicliuni or gill, with supra-branchial cham- 
ber open to and continuous with infra- 
branchial chamber, ib.c. ; /. ligament of 
shell ; in. mantle lobes, lining valves and 
showing marginal pallial muscles ; o.c. outer 
ctenidium; ^c. pericardium ; J>c.g. pericardial 
gland or Keber’s organ : r. rectum, with 
typhlosole ; r.o. renal organ or organ of 
Bojanus, and showing the paired cerebro- 
pleuro-visceral commissures ; sb.c. supra- 
branchial chamber ; u. ureter ; z/. ventricle 
of the heart ; v.c. vena-cava or great central 
blood vessel. 
