246 bobert h. wolcott: 



* In Lake St. Clair (Anchor Bay), of 251 mussels, 62 pe 

 cent infested. 



In Intermediate L., N. Mich., of 52 mussels, 65 per cent 

 infested. 



In Reed's Lake, Grand Rapids, Mich., of 96 mussels, 64.5 

 per cent infested. 



While in Grand River, with a rapid current, collections at 

 various points show: 



Of 175 mussels, 62.33 per cent infested; 

 Of 73 mussels, 60.00 per cent infested; 

 Of 182 mussels, 90.50 per cent infested; 

 Of 85 mussels, 82.00 per cent infested. 



These last two larger percentages were obtained during the 

 past summer, when the percentage of infested mussels was un- 

 usually high. Indeed careful observations made during the 

 summer in the attempt to secure more accurate data than had 

 heretofore been obtained, as to the effect of different conditions, 

 were rendered practically of no value from the fact that the 

 percentage of mussels infested was so abnormally high as com- 

 pared with other seasons that no accurate comparison could be 

 made with former years and other localities. Still further ob- 

 servations are necessary and only general statements can be 

 made in the discussion which follows. 



The mussels which are most sensitive, most active in closing 

 their shells, and whose shells close most tightly, seem to be 

 more immune from the presence of parasites than those possess- 

 ing less of these properties. The following figures have (p. 245) 

 been selected with reference to such species of mussels as have 

 been represented in the collections by a considerable number of 

 specimens and illustrate different types as regards form. 



If these U. occidens and JJ. ventricosus are thick, the valves 

 meet at a considerable angle and not closely, and they are 

 slower to close upon irritation. U. gilhosus, JJ. rectus and 

 JJ. nasutus are long and thin, the valves meet at a small angle 

 and tightly and they are more active. The other forms are in- 

 termediate in these respects. 



