Phylum Molhtsca and Minor Eucoelomate Phyla 475 



receptors in the region of the incurrent siphon, and tactile areas on the 

 edge of the mantle. 



The sexes are separate in Anodonta but are indistinguishable ex- 

 ternally ; the gonads are located around the intestinal coils and are much- 

 branched. The tgg or sperm passes through a short duct opening into 

 the mantle cavity in the region of the kidney. 



Life History of Anodonta. — The sperm are discharged into the 

 water and are brought into the female's mantle cavity with the incur- 

 rent water. The eggs are fertilized in the water tubes of the gills. 

 They then pass into the cavities of the outer gills which enlarge 

 into brood chambers. Each Cigg develops into a small larva known 

 as the glochidium, which has two valves bearing ventral hooks. The 

 valves are closed by an adductor muscle and possess a single long byssus 

 thread. After a period of development, the larvae are released with 

 the water from the dorsal siphon, usually in response to the proximity of 

 a suitable fish. The larvae then attach to the tish ; in some species, they 

 attach to the fish's body ; while in species such as those of Anodonta, they 

 attach to the gills. The larvae become surrounded by cysts of host 



Fig. 155. — Life cycle of the fresh-water clam. a, Clam expelling glochidia 

 when approached by fish; b, glochidia; c, fish with glochidia attached to fins and 

 gills; d, young clams falling off; e, young clams. 



