258 



THE ANIMAL KINGDOM 





Figure 14.12. Glochidium, the larva of a fresh-water mussel. 



for life in lakes and rivers where floating larvae would be swept away. 

 Eggs are retained in the adult until they become small bivalves called 

 glochidia (Fig. 14.12), mostly shell and adductor muscle with very little 

 else. These clamp tightly onto the fins or gills of fish, where they grad- 

 ually become buried and actually receive nourishment. In this way they 

 are carried about, upstream as well as down. In a few weeks the glochid- 

 ium assumes an adult form and ends its parasitic phase by dropping off 

 and burrowing into the bottom. 



The fresh-water mussels of the Mississippi River system support a 

 pearl button industry. The buttons are cut from the inner, pearly 

 layers of the shells. At present the industry is considerably reduced in 

 size, both because many of the fisheries have been depleted and because 

 competition with substitutes has driven the pearl button into a semi- 

 luxury category. So far plastics have failed to imitate the unique luster 

 of pearl, which results from the structure of its crystals, and not from 

 the material of which it is made. 



A few pelecypods lie loosely on the ocean bottom and are able to 

 swim by flapping the shells. An example is the scallop, a species of the 

 genus Pecten (Fig. 14.11). The familiar scalloped shells are closed by an 

 enormous adductor muscle, the only part of the scallop that is eaten. 

 The free edges of the mantle are set with bright blue eyes. Scallops are 

 easily frightened, and violently clap their shells as they swim away on 

 erratic courses. 



1 1 2. Class Scaphopoda 



The tooth shells are a small group of marine molluscs that burrow 

 in mud and sand. They have a funnel-shaped shell open at both ends 

 (Fig. 14.2). The foot is conical and used for digging. Around the head 

 are a number of prehensile filaments that are presumed to be used to 

 bring food particles to the mouth. A radula is present. The smaller open- 



