PART 3. STRUCTURE OF FRESH-WATER MUSSELS. 

 INTRODUCTION. 



A general description of the structure of fresh-water mussels may assist those 

 without special knowledge of the anatomy of mussels to follow intelligently the account 

 of the natural history, propagation, and development which it has been the primary 

 purpose of this report to give. It may also serve as a helpful introduction to persons 

 with limited technical knowledge who wish to make original observations or experi- 

 ments concerning the habits and growth of mussels. It has been the special purpose 

 of the authors to point out the more conspicuous gaps in our knowledge of the behavior 

 of mussels and their relations to the environment. Many of these gaps can readily be 

 bridged by any who will take the trouble to obser\-e painstakingly and repeatedly the 

 conditions under which fresh-water mussels live in the streams, lakes, or ponds in one's 

 own neighborhood. The species subjected to observation or experiment should of 

 course be definitely known, but identifications of species can always be obtained of 

 Government agencies or from independent specialists in the study of moUusks. 



In most localities some species of mussels are easily obtainable and observable 

 in nature or in aquaria. In rivers of the Atlantic States, generally, the common mussel 

 is the Unio complanatus. The more familiar forms in lakes and alongshore in streams 

 of the Mississippi Valley and the Great Lakes drainage are the fat mucket LampsUis 

 lutcola, " and some of the floaters of the genus Anodonta. Closely related to the fat 

 mucket is the mucket, LampsUis ligamentina, which is common in the Mississippi and 

 its tributaries as well as in many streams discharging into the Great Lakes. As a rep- 

 resentative type in the simplicity of its form and of the sculpture and markings of its 

 shell, the mucket serves as the basis of the following general description, except 

 as explicit qualifications are made. With more or less modification, the account may 

 be applied to whatever species is most readily available. The functions of the organs 

 described will generally be briefly indicated. 



Let it be understood first that a living mussel is commonly partly embedded in the 

 bottom, with the forward end directed obliquely downward and the rear end upward. 

 The "mouth" as understood by fishermen is in reality the double siphonal opening 

 in the hinder part of the mussel; the true mouth, through which food is taken into the 

 body, is a very small and scarcely discernible opening in the part of the soft body which 

 is farthest away from the exposed end of the mussel. 



The fresh-water mussels differ markedly in structure from the oyster or the pearl 

 oysters which pertain to a different order of lamellibranchs. They are likewise far 

 removed from the sea mussels, which lie in a third order. Their nearer relatives are 

 the sea clams and the small Cyrenians of the rivers; the sea clams and the little clams 

 (Cyrenians) of the rivers are more closely allied to each other than to fresh-water 

 mussels. The pearly fresh-water mussels or Naiades comprise two great families, 



<i The best commercial type of the mussels of this species is also luiown as the "Lake Pepin mucket." 



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