PHYLUM MOLLUSCA 



249 



from the pericardium by cilia, and other excretory matter is 

 taken from the blood by the glandular portion (kd). These 

 waste products of metabolism are carried out of the body through 

 the dorsal siphon (ex.spli). 



Nervous System. — There are only a few ganglia in the body 

 of the mussel. On each side of the oesophagus is a so-called 

 cerebro pleural ganglion (Fig. 

 175, c.pl.gn), connected 

 with its fellow by a nerve 

 called the cerebral commis- 

 sure which passes above the 

 oesophagus. From each 

 cerebropleural ganglion a 

 nerve-cord passes ventrally, 

 ending in a pedal ganglion 

 (pd.gn) in the foot. The 

 two pedal ganglia are closely 

 joined together. Each cere- 

 bropleural ganglion also 



gives off a cerebrovisceral connective (dotted in Fig. 175) which 

 may be enclosed by the kidneys and leads to a visceral 

 ganglion (v.gn). 



Sensory Organs. — Fresh-water mussels are not well pro- 

 vided with sensory organs. A small vesicle, the statocyst, con- 

 taining a calcareous concretion, the statolith, lies a short way 

 behind the pedal ganglia. It is an organ of equilibrium. A 

 thick patch of yellow epithelial cells covers each visceral ganglion 

 and is known as an osphradium. The functions of the osphradia 

 are not certain. They probably test the water which enters the 

 mantle cavity. The edges of the mantle are provided with 

 sensory cells ; these are especially abundant on the ventral siphon 

 (Fig. 175, in.sph), and are probably sensitive to contact and 

 light. 



Reproduction. — Mussels are usually either male or female; 

 a few are hermaphroditic. The reproductive organs are situated 



Fig. 177. — Transverse section of por- 

 tion of an outer gill-plate of Anodonta. 

 il, inner lamella; il', outer lamella; 

 ilj, interlamellar junctions; v, large ver- 

 tical vessels. (From the Cambridge 

 Natural History, after Peck.) 



