Economic Importance of Anim,als 541 



million years ago) down to the present. These forms are among the 

 earliest in geologic records (Figs. 320 to 322). 



The "sandworm" or "clamworm" of the genus Nereis is used as food 

 by marine animals (Fig. 113), 



Leeches (class Hirudinea) (Fig. 114) are parasitic annelids which 

 infest both vertebrate and invertebrate animals. They have been used 

 as food by certain peoples. Medicinal leeches (Fig. 114) are used to 

 draw blood in such conditions as "black eyes" and after contusions. 

 Leeches produce a substance (hirudin) which prevents the clotting of 

 blood. This enables the leech to secure blood as food after once attach- 

 ing itself to its host. Leeches, because of their soft construction, have 

 left no geologic records. 



PHYLUM 10— MOLLUSC A (OYSTERS, CLAMS, SQUIDS, 

 SNAILS, DEVILFISH, OCTOPUS) 



Geologically, the Mollusca are found from the Cambrian period (550 

 million years ago) to the present time (Figs. 320 to 322). Clams and 

 mussels were especially abundant in the Cretaceous (chalk) period in 

 America. 



Certain types of mollusk shells have been used as money in certain 

 communities. Certain shells are used in button manufacture and are 

 ground for chicken feed or used as fertilizer. Molluscan shells have been 

 and are still being used as ornaments in a great variety of ways. The 

 shells also may be used for road-building purposes. The "window- 

 glass" shell of Placuna placenta (class Pelecypoda) is used as a window 

 pane in certain parts of the tropics. 



In the embryologic development of the fresh-water mussels, the so- 

 called Glochidium stage attaches itself to the gills and fins of fishes, thus 

 ensuring its distribution. Mollusca are especially suited for studies of 

 growth because the shell is added and extended by the mantle as the 

 animal grows. Oysters, scallops, clams, and mussels are used as food by 

 man. Oysters and other Mollusca may be a means of transmitting ty- 

 phoid fever and other diseases unless they are properly grown and trans- 

 ported. Rigid inspection has reduced this possibility to a great extent. 

 Sometimes snails may attack the eggs of fishes in nests, others may de- 

 stroy plants and vegetables, and still others may act as intermediate 

 hosts to various parasites, transferring them to other animals. 



Pearls are manufactured by pearl oysters, mussels, and clams by an 

 accumulation of "nacre" or "mother-of-pearl" laid down in layers around 



