ANATOMY OF THE CLAM. 225 



to tne heart ; 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- 



F'g. 158.- 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 Fead the blood in part directly towards the organ 4, which is the 

 .Sidney 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 oesophagus (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 otocyst 

 (Fig. 160, i), 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 otolithy The structure of 

 this octocyst may be considered typical for Invertebrates. 



