ANODONTA — INTERNAL ORGANIZATIOII. 
The Pedal or foot region is anteriorly i)laced upon the ventral side 
of the body, and is a large laterally compressed and strongly ciliated 
muscular appendage or protuberance, acutely angulated below and 
terminating in a pointed and sensitive tip, bearing on its posterior 
margin three minute pit-like dei)ressions, which are the openings of 
mucous glands, and of the degenerate byssal gland, but have been 
assumed b}' some to be the external oritices of a water vascular 
system. It is capable of protrusion greatly beyond the shell by 
means of blood i)ressure, but is withdrawn by muscular contraction ; 
and is esi)ecially adapted for forcing its way through mud and sand 
by alternate expansion and contraction, being noticeably stronger in 
those forms which habitually plough through or burrow within the 
mud or sand. 
'I’he Visceral sac or region is compressly oval, and (pdte concealed 
by the overlapi)ing mantle lobes, but, as in the (}astroj)oda, it contains 
the liver, the genital glands, the convolutions of the alimentary canal, 
the circulatory organs, the renal organs, etc. ; along each side, above 
the base of the foot, is a vascular longitudinal ridge, from each of 
which depends a pair of long rellected branchial leaflets, which 
originate behind the labial pal[)S and pass backwards ; and whose 
moditicatioiis have been ably used liy Prof Pelseneer as a basis for 
the cla.s.sitieation of the Pelecy})oda. 
Fid. ‘^2 . — Anodonta cyj^ncay with the right mantle lobe removed and pericardium opened to 
show the general and relative arrangement of^ the organs of the body (modified after Howes). 
a. auricle of the heart ; a. ad. anterior adductor; a.s. anal siphon ; br. branchix*, with posterior 
suspensory ligament ; br.s. branchial siphon ; d.o. dorsal orifice, in connection with the anal siphon ; 
/. foot, with the three pit-like depressions on the posterior margin indicated ; /. liver or digestive 
gland ; nt. mouth ; vt.a. line of attachment of mantle to visceral mass ; />. palps ; p.ad. posterior 
adductor ; pc. pericardium ; pc.gl. pericardial gland or Keber's organ ; r.o. renal organ or kidney ; 
7’. ventricle of the heart. 
The Internal ( Irganization of our ty[)e has retained the primitively 
jtaired character that distinguished its hypothetical ancestor, such 
organs as are not symmetrically paired being placed along the medial 
