ANODONTA — REPRODUCTIVE SYSTEM. 183 
are more especially distributed along the anterior margin, some of 
the fibres being also dorsally spread over the region of the liver. 
The Po.sterior retractors are situate dorsally to, but contiguous with 
the posterior adductor, and originate as firm muscular trunks ; the 
fibres into which they are divided become more especially distributed 
about the lower part of the body and along the free ventral edge of 
the foot, and, though generally more deeply placed, are often closely 
intermingled i\ith the more superficially placed protractor fibres. 
The Umboiial reteiitor or Levator muscles are, in Anodonta, formed 
by one or more gToups of more or less isolated fibres fixed near to or 
within the umbonal area, their fibres spreading over the region of the 
stomach and of the pericardium. Many of the fibres do not, how- 
ever, reach the shell, their proximal extremities calcifying beneath 
the mantle. 
The Intrinsic Pedal muscles are very numerous and intricate and 
formed by a complex arrangement of variously directed and inter- 
lacing muscle fibres, the longitudinal ones being continuous with or 
connected to the great retractors of the foot, while the transverse 
muscle bundles and the more superficially placed layer of circular 
fibres assist in its contraction and protrusion. 
The Reproductive organs of Anodonta are normally of a dioecious 
character, the two sexes being developed in different individuals, 
although hermaplu’odite specimens are sometimes found. 
Fig. 354. — Transverse section through the 
anterior part of the pericardium of Anodonta 
cygnca to show the position of the genital 
and nephridial ducts and their relation to the 
supra-branchial chambers of the inner gills 
(after Howes). 
c.pl'P.c. cerebro - pleuro - visceral commis- 
sure, with the Vena cava beneath ; f. foot, 
with contained genital glands and showing 
cross sections of the intestinal tract and more 
longitudinal ones of the subsidiary genital 
ducts ; g.d. genital duct of left side opened 
up to show outlet ; i.c. inner ctenidium, the 
left showing the outlet of the genital and renal 
ducts ; ibx. infra-branchial chamber ; 1. liga- 
ment of shell ; mantle lobes lining shell 
and showing marginal pallial muscles ; o.c. 
outer ctenidium ; pc. pericardium ; pc.g. peri- 
cardial gland or organ of Keber ; r. rectum 
with typhlosole ; r.o. renal organ, glandular 
portion, showing beneath the more dorsal 
efferent renal chamber or ureter ; sb.c. supra- 
branchial chamber ; u. the left ureter showing 
the opening into the supra-branchial chamber 
of the inner gill. 
The geuital glands are very voluminous, symmetrically paired, and 
placed at either side of the body, occupying the upper part of the 
cpl.p 
