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 



