Life of the Cla?ns 39 



A certain amount of respiration may take place even when the valves 

 of certain bivalves are completely shut during exposure to dryness or to 

 heat from the sun. What little air may be trapped within the mantle cavity 

 of the animal is soon used up. Oxygen is then obtained anerobically (with- 

 out contact with air) by cleavage of reserve glycogen substances stored in 

 the clam's tissues. Carbon dioxide builds up and is dissolved in the fluid 

 in the mantle cavity, and the resulting increase in acidity may dissolve or 

 etch away portions of the shell. Shells of oysters and the Patella limpets 

 vv^hich are kept dry on the rocky coast for unusually long periods show 

 considerable etching on the inside. Shells of the Date Mussels {Lithophaga), 

 which live in a small volume of water in their rock burrows, are etched in 

 this manner, while those specimens which live in the same volume of well- 

 aerated water are not etched. 



Bivalves can be forced to cease respiration for several days without 

 succumbing, but they are very susceptible to polluted waters and excess 

 amounts of silt. The "red tide" caused severe destruction to the marine fauna 

 on the west coast of Florida in 1946, and for several years afterward the 

 "shelling" on famous Sanibel Island was little better than it is on Coney 

 Island Beach, New York. "Red tides" have occurred from time to time in 

 California, Washington State, Japan, Austraha and elsewhere. They are caused 

 by an unusual increase in the numbers of single-celled dinoflagellates, Gony- 

 aiilax. It is believed that billions of these organisms not only deplete the 

 oxygen supply but also clog the gills of fish, moUusks and other animals which 

 die in vast numbers and further befoul the ocean. Fortunately, these "red 

 tides" spend themselves out, and the coastal waters return to normal in a 

 few years. 



Another species of Gojiyaulax (G. catanella) may be ingested by mus- 

 sels and clams and, although it does no harm to the mollusk, it is highly toxic 

 to humans who may eat the infected shellfish. A number of deaths have 

 occurred on both of our coasts from this type of mussel poisoning. There 

 is no way of distinguishing poisonous from sound mussels by their appear- 

 ance, and heat does not destroy the poison. Mussel poisoning occurs along 

 the California coast from May 15 to October 15. There is another such 

 center in Nova Scotia. 



Among the various schemes of classification of the bivalves, the type 

 of gill structure has been used by many students of phylogeny (the study 

 of molluscan ancestral trees), such as Lankester, Pelseneer, Ridewood and 

 others. Opponents to this system, such as Neumayr, Munier-Chalmas, Dall, 

 Cotton and others, have based their classification on the hinges of the shell 

 valves. Neither system is without its weaknesses, and in some modern schemes 

 the two systems are employed together. 



There are four main types of gills: (i) Protobranch, in which the gills 



