MOLLUSCA 



259 



some infected or irritated point on the epidermis of the mantle. Great quantities 

 of these pearls are sometimes found in the graves of the mound builders. 



In some of the fresh-water clams there is an interesting temporary parasitism 

 during the early development. When the young, called glochidia, escape from the 

 gills of the mother, they must become attached to the skin or gills of a fish. 

 They may become deeply imbedded and be nourished in the tissues of the host for 

 several weeks. During this time they pass through their metamorphosis and 

 develop the adult organs. This habit brings about a better distribution of the 

 clams than would otherwise happen, since it has been shown that adult clams do 

 not voluntarily migrate widely. 



FIG. 119. 



PIG. 119. Pecten irradians, a Scallop. From Binney's Gould. 



Questions on the figure. Is this an external or internal view of the shell? 

 Where is the umbo? What is peculiar about the hinge in this case? What is the 

 significance of the lines nearly concentric with the margin? Of the radial lines? 



Order 2. Heteromya: Two adductor muscles, the anterior much reduced; 

 siphon usually wanting. Here are included the horse-mussel (Modiola) and 

 Mytilus, edible mussels which occur in clusters just below low tide mark; also the 

 pearl-oyster, from which the best pearls are taken. The last mentioned form is 

 not found on our own coasts. 



Order 3. Monomya: One adductor muscle (posterior); no siphon. The genus 

 Ostrea (oyster) and the genus Pecten (scallop) are the most interesting and impor- 

 tant representatives of this order. The species of Ostrea differ much in size in 

 different regions. The largest living species is a Japanese form which is known to 

 reach a length of two to three feet. The oyster is hermaphrodite. The young 

 after a short free life, become attached by one of the valves. The oyster constitutes 

 a larger element in the food supply of man than any other invertebrate. The 

 scallops are not attached, and swim by a rapid opening and closing of their valves. 



