ANATOMY AND PHYSIOLOGY oF ANODONTA FLUVIATILIS. 181 
sac (Pl. 9, fig. 4, n. g. s.), the roof of which is formed by the floor 
of the pericardium. It is filled with water and is separated from the 
glandular portion by a thin transparent wall. The glandular sac is 
situated immediately below the non-glandular sac, and is filled with 
dark brown, nearly black granular matter (Pl. 9, fig. 4, g.8.). The 
non-glandular sac extends only to the posterior portion of the pericar- 
dium. ‘The glandular sac extends below and in front of the posterior 
adductor muscle, and invests the tendons of the posterior retractor 
muscles, In the floor of the pericardium immediately below the place 
where the intestinal canal enters the body, are two small oval open- 
ings, with tumid lips (Pl. 9, figs. 4, 5, 7 0. p.g.). These openings commu- 
nicate with passages through the glandular sac. (Pl. 9, figs. 4-7, o. p. g.) 
Near the posterior portion of the glandular sac these passages enter 
the non-glandular sac. (Pl. 9, figs. 4, 6,'7,0.e.s.) In the anterior portion 
of the non-glandular sac is a small opening which communicates with 
the epibranchial chamber, just back of the attachment of the inner 
gill to the body. (Pl. 9, figs. 4-7, 0. b. ec.) It will thus be seen that 
the pericardium indirectly communicates with the exterior. This 
arrangement will be more distinctly understood by referring to Pl. 9, 
fig. 7, where the passage is represented as a tube. 
The walls of the vena cava are traversed by numerous small blood- 
vessels which pass from the vena cava to the gills, 
The renal organ is, in all probability, analogous to the kidney of 
vertebrate animals. Its functions will be again spoken of under the 
head of circulation of the blood. 
PERICARDIUM, HEART. 
(Plate 9.) 
From a point on the dorsal portion of the body just posterior to the 
umbo, to the posterior adductor muscles, is an oval cavity about twice 
as long as wide, inclosed above by the thin semi-transparent part of 
the mantle, and filled with a colorless fluid. This cavity is the peri- 
cardium (Pl. 9, figs. 1, 2, 3, p.), and differs in some important re- 
spects from the analogous organ in man and other vertebrate animals 
in which it consists simply of a sac filled with a fluid, facilitating the 
movements of the heart, while in the Anodonta and mollusca gener- 
ally it contains blood as well as other fluid matter, and connects indi- 
rectly with the exterior. The movements of the heart may be observed 
through the thin mantle — when the mantle is removed the heart is 
laid bare. It consists of one median and two lateral chambers, re- 
spectively named the ventricle and auricles. (Pl. 9, figs. 1, 2, 4, v. au.) 
The ventricle is a sac of yellowish color, oval in form, contracting 
