, 
4 
192 MOLLUSCA. 
Mr.Owen. In Orbicula two small ganglia were found on 
the side of the oesophagus next the perforated valve ; from 
which two filaments, accompanying the oesophagus through 
the membranous wall, immediately diverge and pass ex- 
terior to the anterior shell muscles, proceeding with corres- 
ponding arteries to near the heart, beyond which he could 
not trace them. A single small ganglion is situated on the 
opposite side of the oesophagus, but on a plane posterior to 
the preceding; this is probably the cerebral ganglion for 
giving off nerves to the free spiral extremities of the arms, 
close to the base of which it is situated. 
OrveEr II.—BIvALvIA. 
Mouth destitute of fringed spiral arms. 
The animals of this group form the class Conchifera of 
Lamark, the Bivalvia of the older naturalists. 
The shells exhibit great variety of form and relative size. 
They are joined together at the hinge, which is either plain 
or toothed, and corresponds in position with the back of the 
animal. The connection of the two valves is secured by 
the intervention of an elastic horny ligament, the office of 
which is to keep the valves open. It is either external or © 
internal. The valves are closed by means of adductor 
muscles, intermixed with tendons, and, passing traversely 
through the animal, adhere to the corresponding places in 
the inside of each shell. By the contractions of these 
muscles the free edges of the valves are brought into con- 
tact, at the same time that the ligament is compressed or 
stretched, according as it is internal or external. The num- 
ber of muscular impressions is employed by Lamark in the | 
division of the Bivalvia into two orders, Dimyaires and | 
