FRESH-WATER MUSSEL 211 



made up of many thin lamellae. The nacre is thickest at the hinge 

 ligament and becomes gradually thinner toward the margin. The shell 

 is constantly being added to at the margin and thus increased in surface 

 area, and at the same time it is constantly increasing in thickness by 

 deposition from within. 



245. Internal Anatomy. — Inclosed in the shell is an animal which is 

 very soft and which when removed from the shell becomes to a consider- 

 able degree shapeless. It may be described as made up of a body mass 

 and foot, a mantle, and two pairs of gills (Fig. 118). The relation of 

 mantle and gills to the body mass is similar to that which a man's clothes 

 would have to his body if his coat, corresponding to the mantle, were 

 grown to his back, and he had on two vests, unbuttoned in front and 

 attached to the sides of his body, corresponding to the gills. 



246. Body Mass and Foot. — The body mass, or visceral mass, is a soft 

 mass filling the upper part of the space between the two valves and is 

 continuous externally with the mantle. Ventrally it becomes narrowed 

 and hangs down in the mantle cavity (Fig. 118), its ventral muscular 

 margin forming the foot. In this body mass are contained various 

 organs, including those of the digestive, circulatory, excretory, and 

 reproductive systems (Fig. 119). 



247. Mantle. — The mantle lines the inner surfaces of the valves of the 

 shell but it extends a short distance beyond their edges and forms, with 

 the periostracum, a soft margin. This mantle secretes the carbonate of 

 lime which is continually added to the edge of the shell and to its inner 

 surface. The space inclosed by the two halves of the mantle is known 

 as the mantle cavity. 



248. Gills. — The gills, which with the mantle carry on respiration, are 

 platelike and are in pairs on each side of the body mass, to which they 

 are attached. Each gill is composed of two lamellae joined all around the 

 margin and also connected by a number of cross partitions known as 

 interlamellar junctions, which divide the inclosed space into a large number 

 of compartments known as water tubes. Each lamella is a sort of mesh- 

 work made up of vertical ridges, known as gill filaments, connected by 

 horizontal bars. Some of these meshes possess openings which permit 

 water to pass through into the water tubes. The gill filaments are 

 strengthened by chitinous rods. The water tubes open above each gill 

 into a passageway known as a swpr abranchial chamber. The base of the 

 inner lamella of the inner gill is attached to the body mass from the 

 anterior margin backward for a distance which varies in different types of 

 mussels. The attachment is short in those known as anodontas and 

 long in unios. Where the lamella is free there is left a narrow slitlike 

 passage which leads from the mantle cavity directly into the inner supra- 

 branchial chamber on each side. Since these two inner lamellae meet 

 behind the body mass and continue onward posteriorly, the two inner 



