152 



PRACTICAL ZOOLOGY 



The glochidium has a shell (Fig. 86, A, sti) consisting of two 

 valves which are hooked; these may be closed by a muscle (ad) 

 when a proper stimulus is applied. A long, sticky thread called 

 the byssus (by) extends out from the center of the larva, and 

 bunches of setoe (s) are also present. 



In the mussel Anodonta, the eggs are fertilized usually in 

 August, and the glochidia which develop from them remain in 

 the gills of the mother all winter. In the following spring they 

 are discharged, and if they chance to come in contact with the 

 external parts of a fish, this contact stimulus causes them to 





Fig. 86. — A. A young mussel or glochidium. ad, adductor muscle; by, 

 byssus; s, setas ; sh, shell. (After Balfour.) 



B, the gills of a fish in which are embedded many young mussels forming 

 " blackheads." (After Lefevre and Curtis.) 



seize hold of the fish's gills by closing the valves of their shell. 

 The glochidium probably chemically stimulates the skin of the 

 fish to grow around it, forming the well-known " worms " or 

 " blackheads " (Fig. 86, B). While thus embedded, the glo- 

 chidium receives nourishment from the fish and undergoes a stage 

 of development (metamorphosis), during which the foot, muscles, 

 and other parts of the adult are formed. After a parasitic life 

 of from three to twelve weeks, within the tissues of the fish, the 

 young mussel is liberated and takes up a free existence. 



One result of the parasitic habit of larval mussels is the disper- 



