ANATOMY OF THE CLAM. 225 
‘to tne hearts; one end is secretory, lamellar and glandular, 
communicating with the pericardial cavity, while the other 
is excretory and opens into the cavity of the gill, The 
nervous system can be, with care and patience, worked out 
in the clam or fresh-water mussel. In the clam (Mya arena- 
Fig. 168. Circulatory system of Anodonta, a fresh-water mussel, after Bojanus, 
1. ventricle; 2, arterial system; 14 and 15, veins which follow the border of the 
mantle, The veins lead the blood in part directly towards the organ 4, which is the 
xidney or ‘‘organ of Bojanus,’’ and in part to the venous sinus of the upper surface 
of this organ; 5, veins which carry back the blood from the gills, the rest going to 
the sinus, 6, where arise the branchial arteries; 7, 8, the branchial veins, and 9, the 
gill.—From Gervais et Van Beneden, 
ria, Fig. 159) it consists of three pairs of small ganglia, 
one above (the ‘‘ brain”) and one below the esophagus (the 
pedal ganglia) connected by a commissure, thus forming an 
cesophageal ring; and at the middle of the mantle, near the 
base of the gills, is a third pair of ganglia (parieto-splanch- 
nic), from which nerves are sent to the gills.and to each 
division of the siphon. This last pair of ganglia can be 
usually found with ease, without dissection, especially after 
the clam has been hardened in alcohol. ‘The ear of the clam 
is situated in the so-called foot; it bears the name of otocys¢ 
(Fig. 160, 7), and is connected with a nerve sent off from the 
pedal ganglion. It is a little white body found by laying 
open the fleshy foot through the middle. Microscopic ex- 
amination shows that it is a sac lined by an epithelium, rest- 
ing on a thin nervous layer supported by an external coat of 
connective tissue. From the epithelium spring long hairs; 
the sac contains fluid and a large otolith. The structure of 
this octocyst may be considered typical for Invertcbrates. 
