PHYLUM MOLLUSCA 



297 



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Figure 186. 1 he common octopus is a ccphalopod without a shell. It pulls itself over the 

 rocks with its arms or it can move by expelling water from its funnel. The sucker-bearing arms 

 are used to seize the animals on which it feeds. (Courtesy of D.P. Wilson, Plymouth, England.) 



the oyster's gills into little ciliated spheres, 

 called spat. 



Pearls are sometimes found in our edible 

 oysters, but these are not nacreous, and 

 therefore of little value. The most valuable 

 pearls come from pearl oysters, which are 

 not closely related to the edible oyster. The 

 pearl is the result of an injury to the mollusk 

 caused by either an organism or a foreign 

 particle which embeds itself in the fleshy 

 part of the bivalve and causes an irritation. 

 The irritation stimulates the animal to de- 

 posit layer upon layer of nacre around the 

 intruding body to form the pearl; the amount 

 of deposition is in direct proportion to the 

 degree of irritation. The Japanese produce 

 artificial pearls by inserting a foreign body 

 into the mantle of the bivalves which are 

 kept in cages until pearls are produced. The 

 mollusk requires from 3 to 4 years to form 

 a pearl of considerable size and 7 years to 



form a large one. Because the inner layer 

 of the shell is composed of the same nacre- 

 ous substance as the pearl, it is called 

 mother-of-pearl. 



ORIGIN AND RELATIONS 

 OF THE MOLLUSCA 



The Monoplacophora appear to be the 

 most primitive class of mollusks and have 

 changed the least from the ancestral condi- 

 tion. The gastropods have changed to a 

 short creeping type, with a spiral visceral 

 hump revolved through an angle of 180°. 

 The pelecypods separated from the rest of 

 the phylum at an early date; they became 

 flattened laterally and developed a large 

 bilobed mantle that secretes a shell of two 

 valves, a large mantle cavity containing gills, 

 a burrowing foot in place of the creeping 



