FRESH-WATER MUSSELS. I 29 



ratio) to the thickness of shell of the species as strongly to suggest that the limiting 



factor of growth ordinarily is not organic food, but the mineral content of the water 



(p. 87). 



PRESENCE OF SO-CALLED GROWTH RINGS. 



The ages of animals may not infrequently be determined, at least approximately, by 

 the "rings of growth," on teeth, scales, scutes, or otoliths (ear stones), or other hard parts 

 of the body. A similar criterion of age determination is of course commonly applied to 

 trees. More recently the rings on the scutes of terrapin and those on the scales and 

 otoliths of fish have been used for the same purpose. 



This method of determining age is generally based upon the belief that the cessation 

 or the slowing down of growi:h during the winter season may cause the formation of a 

 distinguishable line or band on a concentrically growing structure. By counting the 

 number of winter lines or bands the number of winters through which the animal has 

 passed is ascertained, or by counting the number of zones between such rings, beginning 

 with the center zone, the number of seasons of growth is discovered. It is one thing to 

 know that such rings are formed in winter, but quite another thing to learn just how or 

 why the rings are formed. It is also of primary importance to determine whether or not 

 similar rings may be formed upon any other occasion than the occurrence of a season of 

 winter. In. the case of the fresh-water mussel shell, at least, these questions can be 

 answered by observations and experiments. (Coker, unpublished notes.) 



Some years ago when collecting mussels in lakes in southern Michigan it was ob- 

 served that the shells of the fat muckets were all marked wdth several conspicuous rings 

 which were approximately equally spaced on all the mussels of a bed. It seemed a 

 natural inference that these dark rings represented winter periods and thus afforded a 

 means of age determination. At another time, upon examination of mussels which 

 had been measured and placed in crates in the river two years previously, it was observed 

 that there were rings apparently corresponding to the two winters which had elapsed 

 since the date of original measurement, but that there was also another ring which 

 marked the exact size of the mussel when originally measured. (See text fig. 6.) 

 Subsequent obsen.'ations showed that whenever a mussel was measured and replaced in 

 the water, a ring would be formed on the shell before growth in size was resumed. 



These observations led to an effort by microscopic examination of sections of the 

 shell to determine the significance of rings which apparently could be formed either by a 

 season of cold weather or bj' the procedure of taking a mussel from the water, applying a 

 caliper rule, and returning it to the water. To make clear what was learned from the 

 study of the sections it is necessary first to explain briefly the mode of formation of shell 

 which leads to growth in size. 



MODE OF FORMATION OF SHELL. 



The shell is composed of four distinct layers (text figs. 1,2, and 3). The outer is the 

 horny covering called the periostracum.*^ Immediately beneath this is a calcareous 

 layer composed of prisms of calcium carbonate set vertically to the surface. This pris- 

 matic layer is very thin, though thicker than the periostracum, and is likely to remain 



a The fact that the periostracum itself comprises 2 layers of separate origin, while very significant in some respects, is imma- 

 terial in this connection. 



