Anatomy and Physiology of Anodonta Fluviatilis. 183 



The inner lamina of the outer gill and the outer lamina of the inner 

 gill are attached. The inner lamina of the inner gill are at the an- 

 terior portion attached to the foot, but soon become free and remain 

 so for about one-half their length; back of the adductor muscle they are 

 united, and the dorsal part of the gills form a partial floor across 

 the space between the two lobes of the mantle, separating that space 

 into the branchial and epibranchial chambers. The two lamina of 

 each gill are united by their plates. There are about sixteen of these 

 plates in the space of five millimetres. 



It; is in the spaces or pockets formed by these plates and the laminae 

 of the gills, in the outer gills, that the young of the Anodonta remain 

 after being expelled from the ovaries, until they arrive at a certain 

 stage of development. The outer gills when rilled with young are very 

 much distended, the thickness being several millimetres. The form of 

 the young sfiells can be distinguished only by the aid of a lens, and 

 in their form they differ so much from the parent, that to a person 

 who had not read a description of or critically studied them, their true 

 character would not be apparent. 



The outer face of each lamina is composed of flat plates (PI. 7, fig. 

 1, g.p.) supported or rendered firm by chitinous rods (PI. 7, fig. 1, ch.), 

 cylindrical in shape, two rods in each plate. Apparently these rods 

 are short, regularly arranged in pairs, with a short space between each 

 pair without rods, but on close examination, they will be seen to be con- 

 tinuous though at regular distances very slender. The edges of these 

 plates have large cilia (PL 7, fig. 1, ci.), which keep up a constant 

 motion. These plates support on one side a mesh-work of capil- 

 laries (PI. 7, fig. 2), arranged in bands, the space between each band 

 equal to the width of the band. The gills possess nerves which will 

 be described under the head of the nervous system. 



Musculae System. 

 (Plate 8.) 



The principal muscles are the anterior and posterior adductors, the 

 anterior and posterior retractors, and the protractor pedis ; in addition 

 to these there are two small muscles near the umbo, and small muscles 

 along the pallial line. 



The anterior and posterior muscles are cylindrical bundles of fibres, 

 which pass transversely from one valve to the other, and serve to keep 

 the valves closed, antagonizing the action of the ligament. Whenever 

 from the will of the animal or from any cause the muscles of the 

 animal are relaxed, the valves open by the action of the ligament. 



The anterior adductor (PI. 8, a. a.), is situated near the anterior mar- 



