PART 1. NATURAL HISTORY OF FRESH-WATER MUSSELS. 



HABITS. 



CONDITIONS OF EXISTENCE. 



A mussel, in natural position in a stream, is partly or almost entirely embedded 

 in the sand, mud, or gravel of the bottom (PI. V, fig. i). Almost invariably it will be 

 found to have an oblique position, the front end of the body being directed down into 

 the bottom and in a direction with the flow of the current, while the hinder end of the 

 shell is exposed and is directed upward and against the flow of the stream. Unless the 

 mussel has been disturbed, the shell will be slightly gaping, with the edges of the mantle 

 protruding through the opening and closing it everywhere except at the rear (upper) 

 end where it is so arranged as to form two neat funnel-like openings. The upper open- 

 ing is usually the smaller, and the edges of the mantle about it are smooth or crinkled. 

 The lower opening is generally much longer, and the border of the mantle here is com- 

 monly adorned with a number of delicate feelers, or water testers as these may be called 

 (PI. V, figs. I and 2). The significance of the two openings can be easily ascertained 

 if a small amount of some colored liquid, such as finely powdered carmine in water, is 

 placed near to the openings in a mussel which has been allowed to remain undisturbed 

 in a small aquarium or dish of water. The carmine may be seen to be expelled forcibly 

 from the upper smaller opening, while, if placed near the lower opening, it will be drawn 

 in. It becomes apparent that the water is continually drawn in through the lower 

 (inhalent) opening and passed out through the upper (exhalent). In view of the func- 

 tions of the gills and the mantle, described on page 174, it may be understood that this 

 stream of water not only serves the purpose of respiration but also that, as it is strained 

 through the minute pores in the surfaces of the gills, it must yield up the microscopic 

 materials that serve as food for the mussel. The position of the mussel, directed against 

 the flow of the river, not only insures a more effective resistance should the current of 

 the river be excessively strong, but it perhaps gives the mussel greater advantage in 

 collecting the food floating with the current. In lakes where no regular current pre- 

 vails mussels may lie with their axes in any direction, but the oblique position in the 

 bottom is virtually constant for those that are not in movement. 



The advantages of rivers over lakes for the growth of mussels may readily be in- 

 ferred. The mussel can draw in and strain onlj' the water that is close about it, and in 

 the quiet water of a lake or pond new supplies of food are brought to its vicinity only 

 by the comparatively slow forces that cause the intermingling of the waters of the lake. 

 In the steady current of a river, on the other hand, the same water is never strained 

 twice by the same mussel and, besides, the action of the current tends to stir up the 

 small organisms and nutritive sediment which abound in the surface scum of the bottom. 

 Observations by Clark indicate that mussels in lakes feed more largely upon plankton 

 than those in rivers, the latter of which contain in their stomachs chiefly detritus or 



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