632 



EVOLUTION OF ANIMALS 



Part V 



Ecological and Economic Importance. Marine mollusks are far more nu- 

 merous than terrestrial ones. Their free-swimming ciliated larvae abound in 

 the surface plankton that forms the basic food supply of the sea. Myriads of 

 pteropods often crowd the surface waters. They are snails, many no longer 

 than cloves, with lobes of flesh that give them their name sea butterflies and 

 enable them to flit and glide on the surface as their namesakes do in air (Fig. 

 31.2). Vast schools of them swim among the icebergs around Greenland and 

 are strained from the water by the whalebone whales. 



Hosts of small snails live on the seaweeds between the tide lines and rasp 

 off the tissue with their filelike tongues. Each incoming tide brings more sea- 

 weeds, inhabited by more snails and with each ebb tide leaves a new harvest 

 for the gulls and sandpipers. In ponds and lake shallows, snails forage chiefly 

 on the plants but from any submerged surface they scrape bacteria, protozoans, 

 and algae. Benefiting by this food they eventually furnish their own bodies to 

 the frogs and water birds. 



The majority of mollusks are hampered by their shells and do not travel far 



BUTTERFLIES 



A. Clione limacina 

 Chief food of 

 Greenland whale 



Fig. 31.2. Pteropods, the sea butterflies, are winged snails, many of them but 

 little longer than cloves. Each side of the foot is extended into a wing and they 

 skip and sail in vast schools on the surface of the sea. One of them {Clione 

 limacina) is the chief food of the Greenland whalebone whales. (Courtesy, Miner: 

 Fieldbook of Seashore Life. New York, G. P. Putnam's Sons, 1950.) 



