Anatomy and Physiology of Anodonta Fluviatilis. 181 



sac (PI. 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 (PI. 9, fig. 4. g. s.). 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 (PI. 9, figs. 4, 5, 7 o. p. g.). These openings commu- 

 nicate with passages through the glandular sac. (PI. 9, figs. 4-7, o. p. g.) 

 Near the posterior portion of the glandular sac these passages enter 

 the non -glandular sac. (PI. 9, figs. 4, 6, 7,o. 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. (PI. 9, figs. 4-7, o. b. c.) It will thus be seen that 

 the pericardium indirectly communicates with the exterior. This 

 arrangement will be more distinctly understood by referring to PI. 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 (PI. 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. (PI. 9, figs. 1, 2, 4, v. au.) 

 The ventricle is a sac of yellowish color, oval in form, contracting 



