BIVALVE MOLLUSCS 333 



to the dorsal side, traverses the heart (see below), and runs back 

 over the posterior adductor to its termination in the upper section 

 of the mantle-cavity. 



Circulatory Organs (fig. 192). — The heart, situated in a 

 pericardial cavity, has the dorsal situation characteristic of Inverte- 

 brates, and is essentially similar to the heart of the Ormer (see 

 p. 308), consisting as it does of a central ventricle to which a 

 thin-walled auricle is attached on either side. Purified blood is 

 received by the auricles from the mantle-lobes and gills, and then 

 passes into the ventricle, which distributes it to the body. 



Respiratory and Excretory Organs (fig. 192). — As already 

 mentioned, the function of breathing is carried out by mantle- 

 lobes and gills. As to excretion of nitrogenous waste, this is 

 effected by two elongated brown kidneys underlying the peri- 

 cardium, with which they communicate on the one hand, while 

 they open to the exterior on the other. 



Nervous System and Sense Organs (fig. 192). — The central 

 nervous system consists, as in the Ormer (see p. 310), of a nerve- 

 ring and a nerve-loop. The former presents a ganglion on each 

 side of the mouth (equivalent to one of the brain ganglia of the 

 Ormer with a lateral ganglion fused with it) connected with 

 one another above and with a pair of foot-ganglia embedded in 

 the body close to the muscular foot. The nerve-loop is connected 

 in front with the upper ganglia of the ring, and its posterior end 

 lies just below the posterior adductor, where it is thickened into 

 a pair of visceral ganglia. The three pairs of ganglia send out 

 nerves to the parts of the body in their neighbourhood. 



The Mussel undoubtedly possesses the sense of touch, espe- 

 cially as regards the edge of the mantle. It is also probable, 

 though not absolutely certain, that it is endowed with smell and 

 taste, and water -testing organs (osphradia) can be recognized 

 near the visceral ganglia, though there is some doubt about 

 their function, for they lie in the upper section of the mantle- 

 cavity in the course of the outgoing currents of water, which is 

 'not in accordance with their supposed function. So-called organs 

 of hearing are present, as in the Ormer (see p. 310), in the form 

 of two little vesicles connected with the foot-ganglia and containing 

 particles of carbonate of lime. They are probably concerned 

 with the sense of equilibrium. Eyes are altogether absent. 



Lamellibranchs are divided into five orders based on the 



