THE MOLLUSCA 



101 



PIG. 83. Larva of 

 clam Anodonta. a, 

 adductor muscle; b, 

 byssus thread; s, sen- 

 sory hairs; t, teeth 

 for attachment to 

 host. 



There is another pair of commissures extending from the 



brain to a pair of pedal ganglia in the foot. 

 A considerable part of the compact 



body of the clam is made up of the 



sex organs which discharge their cells 



near the opening of the kidney. The 



eggs when set free fall into the spaces 



between the lamellas of the gills where 



they undergo their early development; 



the young larvag are then carried out 



through the excurrent siphon and live 



for some time at the bottom of some 



body of water. For the next stage of 



its development the young clam is de- 

 pendent upon becoming attached to the gills or fins of some 



fish. When this opportunity 

 presents itself the young clam 

 closes the valves of its shell over 

 the tissues of its host and hangs 

 there. Later it becomes more 

 or less completely overgrown by 

 the surrounding tissues of the 

 fish, much as a gall insect is en- 

 closed in the tissues of a plant 

 gall. Finally, the young clam 

 breaks out of its enclosure, settles 

 down in the mud, and begins the 

 regular routine life of its parents. 

 The shells of fresh- water clams are 

 much used in the making of but- 

 tons. Occasionally they yield 

 pearls of considerable value. A 



pearl is a calcareous secretion of the mantle which accu- 

 mulates around some foreign body, commonly a parasitic 



PIG. 84. Pearl oyster 

 from Ceylon, showing pearls 

 on inner surface of shell. 



