FRESH- WATER MUSSELS. 173 



varies in different species and under different conditions but is generally under 20 pul- 

 sations per minute. The heart will continue to beat a long time after the shell has been 

 opened. Near the anterior adductor is a greenish mass of tissue, the so-called liver 

 or digestive gland, surrounding the white stomach. Through the transparent tissue, 

 covering the chamber inclosing the heart, another portion of the alimentary tube is 

 generally distinguishable. This is the rectum or hinder portion of the intestine which 

 passes directly through the heart to discharge just above the posterior adductor muscle. 

 The brownish tissue beneath the heart represents the organ of Bojanus, as it is called, 

 with functions corresponding to a kidney. 



To distinguish other organs the mantle must be folded back. The muscular mass of 

 plowshare form and brownish white in color, constituting the anteroventral border 

 of the body, is the foot. Several curtainlike flaps are conspicuous. Toward the forward 

 end are two large earlike flaps, the labial palpi or lipfolds. They are easily torn in folding 

 the mantle back, but if in good condition, it may seen that each of these palps is contin- 

 uous, around the front end of the body, with the palp of the opposite side. Immediately 

 in front of the body they are very narrow and lie one above and the other just below an 

 exceedingly small opening, the mouth, which can be seen only by very careful exami- 

 nation. 



The other two folds are much larger and rounded below. These are the gills, which 

 extend from the anterior third of the body to the extreme posterior end. The inner is 

 slightly the larger. The outer gill is connected above and on the outside to the mantle. 

 Folding this one back, it is seen that it is attached also to the inner gill above. The inner 

 gill on the inner side is attached to the body and, behind the body, to the inner gill 

 of the opposite side. In many species the inner gill is partially free from the body. 

 These gills, though thin, are really basketlike structures, containing chambers within, 

 as will be described below. 



INTERNAL STRUCTURE. 



It is not the province of this paper to enter minutely into the internal anatomy. 

 But the following epitomized statement of the structure of the animal is given to serve 

 as a key to the understanding of the functions of the organism as a whole. 



The digestive system comprises the mouth, with a short tube or gullet, leading 

 from the mouth to the stomach; the dark brown digestive gland, or so-called liver, 

 which surrounds the stomach; and the intestine, which is a long tube that leads down- 

 ward from the stomach and coils upon itself behind the foot in a complex way, before 

 bending upward to approach the back and extend posteriorly straight through the heart 

 as the rectum, which opens just above the posterior adductor muscle. A long, slender 

 flexible gelatinous rod, the crystalline style, is frequently found in the intestine; it 

 serves a function in separating food from foreign particles and comprises a store of 

 enzymes or ferments for use in the processes of digestion (Nelson, 191 8). 



The excretory system comprises a functional kidney with a bladder which discharges 

 into the cavity surrounding the heart. 



The circulatory system includes, as in higher animals, heart, blood, arteries, and 

 veins. The blood of a mussel is colorless but maintains a regular circulation from the 

 heart through certain arteries to many smaller vessels ramifying all through the body, 

 returning by a main vein to the kidneys, thence to the gills and back through other 

 veins to the heart to begin its course anew. The blood, however, which passes from 

 75412°— 22 12 



