MOLLUSCA 71 



a commissure which lies anteriorly to the oesophagus. They 

 supply the palps and neighbouring parts, including the anterior 

 muscles. Each ganglion is connected also to the pedal and 

 the visceral ganglia by commissures. The former is easily 

 followed, for it runs straight from the one ganglion to the 

 other. It is accompanied by a nerve supplying the otocyst 

 or balancing organ, a pair of which occur just behind the 

 conjoined pedal ganglia. The connective between the cerebral 

 and the visceral ganglia runs upwards and backwards after 

 leaving the cerebral ganglion, and then pursues a straight course 

 to the visceral ganglion. It 

 comes prominently into view as 

 it passes on the inner side of the 

 dark kidney. 



Reproductive Organs. The 

 sexes are separate. The ovaries, 

 as also the testes, occupy the 

 visceral part of the foot. The 

 alveoli fuse to form ducts which 

 merge in a common duct on each 

 side, the external opening of which ,, 



_. '. , , i . FlG - 35. Anodonta. Glochidmm 



lies in the Cavity of the inner gill larva from external gill. The 



below the opening of the coelomi- attaching thread which is in- 



__, dicated only in its proximal 



duct. The Spermatozoa are portion, depends medianly. 



carried out of the mussel by the 



exhalent current and gain entrance to the females by the inhalent 

 currents. The eggs are passed into the chamber of the outer 

 gill, where they are fertilised and develop into larvae, styled 

 glochidia, and the gill, during the season of reproduction, is dis- 

 tended by their presence. The glochidia are provided with 

 bivalve armed shells, an adductor muscle, and the small foot 

 secretes a sensitive, adhesive thread. The glochidia are dis- 

 charged when fish are near, and attach themselves to the body 

 or the fins. The thread contracts and the glochidia become 

 attached firmly to the skin, a tumour or cyst resulting in 

 which the development is completed. When the adult features 

 of structure have been acquired, the young mussel leaves the 

 fish and falls to the bottom to begin its adult life. 



General Considerations. Structurally and physiologically 



