THE FRESH-WATER MUSSEL 



Phylum Mollusca Class Pelecypoda 



I. BEHAVIOR AND EXTERNAL FEATURES 

 Exercise 1. — General Activities. 



(a) Many genera and hundreds of species of fresh-water mussels, 

 or clams, are to be found in the streams, lakes, and ponds of the 

 Mississippi Valley and elsewhere. These mussels live partially em- 

 bedded in the sand or mud of the bottom and crawl about by means 

 of a muscular foot, which can be protruded between the two valves 

 of the shell. Living specimens and empty shells should be collected 

 and brought to the laboratory for comparison with preserved material. 



(b) Observe living mussels in aquaria or shallow dishes containing 

 water and enough sand to permit the animals to bury themselves. 

 Note the normal positions assumed when at rest and when crawling. 

 Does the animal bury itself completely? Note the openings, or 

 siphons, which lie between the valves at the end that is exposed when 

 the mussel is buried. With a pipette introduce powdered carmine or 

 similar particles into the water near this end of the animal. The 

 functions of the currents thus demonstrated will be understood after 

 an examination of the internal structure. With a glass rod touch the 

 margins of the two siphons and observe the result. Can you detect a 

 difference in the sensitivity of these two openings? Are there papillae 

 on the siphonal margins? What is their function? Place a dish con- 

 taining an active mussel in direct sunlight or under a strong light. 

 When the animal has expanded pass a shadow over the siphons to test 

 their sensitivity to light. Insert the end of the glass rod between the 

 two valves. In what way is the response a protective reaction? The 

 fleshy material, of which the siphons are composed and which lies 

 between the free margins of the valves, consists of right and left parts 

 adhering to the inner surfaces of the valves and is the edge of the 

 so-called mantle. 



Exercise 2.— The Shell. 



(c) Examine the two valves of a shell. In life they are united 

 by a hinge. Alternate periods of rapid and slow growth produce the 



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