FRESH-WATER MUSSELS 



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pads. In this condition the marsupial pouches might be compared to pods filled with 

 closely packed beans, the individual beans representing not single eggs but separate 

 masses of eggs. 



When the tubes of a mature female mussel are empty the gills may be as flat as 

 those of the males, or they may appear as sacks with thin translucent walls. The lat- 

 ter condition generally characterizes the long-term breeders, in which the portions of 

 the gill intended to receive the eggs are permanently enlarged. 



The marsupia are conspicuously colored in some species, but in different species 

 the coloration is not necessarily attributable to the same cause. In the niggerhead, 

 Quadrtda ebcnus, the pig-toe, Quadrxila undata, and other species, the bright-red appear- 

 ance of the marsupia is due to the deeply colored eggs showing through the thin walls 

 of the marsupia. In the yellow sand-shell, Lampsilis 

 anodonioides , the pocketbook, Lampsilis vcniricosa, 

 and the Lake Pepin mucket, Lampsilis lulcola, the 

 pigment lying in the outer walls of the ovisacs takes 

 the form of dark bands on the lower portion of the 

 marsupium, the pigmentation becoming more dense 

 and conspicuous when the mussels are gravid. In the 

 young Lampsilis ellipsijormis that we have seen the 

 pigmentation is more intense and more general, ex- 

 tending even to the upper portion of the marsupia, 

 but there restricted to the partitions separating the 

 ovisacs. The color in the black sand-shell, Lampsilis 

 recta, and the Missouri niggerhead, Obovaria ellipsis, is 

 white or cream, in contrast to the yellowish color of 

 the remainder of the ovisacs. 



The extent to which the gills are specialized or 

 modified to receive and retain the eggs while they are 

 developing into the glochidia has been largely utilized 

 in the classification of mussels. All of the North 

 American species belong to the groups in which the 



brood pouch or marsupium comprises either all four gills or only the outer gills. 

 This group, in turn, is divided into the following seven divisions, according to the spe- 

 cializations involved (Simpson, 1900, p. 514): 



1. Marsupium occupying all four gills, as in the niggerhead mussel, Quadrula ehenus, and perhaps 

 all Quadrulas (PI. XIII, fig. i). 



2. Marsupium occupying the entire outer gills, as in the heel-splitter, Symphynota complanala (PI. 

 XXI. fig. i). 



3. Marsupium occupying the entire outer gills, but differing from the second in that the egg masses 

 lie transversely in the gills, as in the squaw-foot, Slrophiius cdctilulus. 



4. Marsupium occupying only the posterior end of tlie outer gills, as in tlie black sand-shell, Lamp- 

 silis recta, etc. (PI. XIII, fig. 2). 



5. Marsupium occupying a specialized portion in the middle region of the outer gills, as in the 

 three-homed wartj'-back, Obliquaria rcfiexa (PI. XIII, fig. 3). 



6. Marsupium occupying the entire lower border of the outer gills in the form of peculiar folds, as 

 in the kidncy-shcU, Ptychohranchus pimseolus (PI. XIII, fig. 5). 



7. Marsupium occupying the lower border only of the outer gills, but not folded, as in the drome- 

 dary mussel, Dromus dramas (PI. XIII, fig. 4). 



Most of the commercial species belong to the first and fourth types. 



Fig. 7. — Horizontal section of a water tube o( a 

 gravid marsupium, showing respiratori' canals 

 (r.c.). and marsupial space (m. j.), containing 

 glochidia. (.\fter Leie\'rc and Curtis.) 



