254 



ZOOLOGY" 



and pearls of value are not of infrequent occurrence. These are merely 

 the mother-of-pearl, which ordinarily lines the shell, secreted about a grain 

 of sand or other irritating object which finds its way between the mantle 

 and the shell. Great quantities of these pearls are sometimes found in 

 the graves of the mound builders. 



FIG. 116. 



FIG. 116. Mytiius edulis, a Mussel. From Binney's Gould. 



Questions on the figure. Identify the umbo. What are your evi- 

 dences that it is the umbo? Compare the lines on the shell with those in 

 figure 117. What is the significance of the specific name (edulis) ? What 

 are the habits of the species? 



Order 2. Heteromya : Two adductor muscles, the anterior much re- 

 duced ; siphon usually wanting. Here are included the horse-mussel 

 (Modiola) and Mytiius, 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 important 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 oiher invertebrate. The scallops are 

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



Class II. Gasteropoda (Snails, Slugs, Whelks, and Periwinkles'). 

 Gasteropods are mollusks with unsymmetrical, univalved, usually spiral 

 shells (occasionally lacking the shell altogether). The head and foot 



