MOLLUSCA 159 



Many of the snails respire by means of air taken into the mantle 

 cavity which functions as a lung. Other aquatic forms breathe by 

 means of gills. Pinna-globulin is a pigment in the blood of the 

 Lamellibranchs. Pinna squamosa has manganese instead of the 

 copper common to invertebrates, including oysters, but it does not 

 appear to function in transporting oxygen. 



Many gastropods and cephalopods have haemocyanin, and in 

 the Gastropod Planorbis, haemoglobin occurs. (Redfield, personal 

 communication.) 



Nervous System. — The nervous system of the MoUusca consists 

 of cerebral, visceral and pedal ganglia, with connectives. In the 

 Gastropoda, the coiled shell causes the nervous system to be in a 

 spiral. 



It is said that the smallest snail can withstand more strychnine 

 than an adult man. Richards has shown « that Mytilus is poisoned 

 readily by atropine and camphor and less so by caffeine. 



Regeneration. — Autotomy is not characteristic of moUusca in 

 general, but a few Lamellibranchs and Gastropods are able to part 

 with and regenerate a new bit of their foot. 



Growth Studies on the MoUusca. — Molluscs are especially 

 suited for studies of the growth of animals because the shell is added 

 to and extended by the mantle as the organism grows. The amount 

 added in a given time, or the rate of growth, depends on the amount 

 of food that the animal receives. During the winter the animals 

 get little food and the edge of the shell thickens leaving a growth 

 ring or check mark when the growth begins again early the next 

 spring. 



On Cape Cod, Massachusetts, the growth of the edible mussel 

 {Mytilus) begins in March following the great increase of its food 

 (plankton) in January and February .^ The growth rings of the 

 Pacific Coast razor clam {Siliqua) appear very clearly so that 

 Weymouth i" and his associates have been able to extend our views 



8 Richards, O. W. 1929. Conduction of the nervous impulse through the pedal 

 ganglion of Mytilus. Biol. Bull., vol. 56, pp. 32-40. Richards has shown that the 

 rate of conduction of the nervous impulse was 92.9 ± 2-2, cm. per second in Mytilus 

 edulis at 24° C. 



9 Richards, O. W. Studies on the growth of M. edulis and calijornianus in progress 

 communicated personally to the author. 



10 Weymouth, F. W., and McMillin, H. C. 1930. Relative Growth and Mortality 

 of the Pacific Coast Razor Clam and Their Bearing on the Commercial Fisheries. 

 Bull. U. S. B. F., vol. 46, pp. 543-567- 



