ANATOMY AND PHYSIOLOGY OF ANODONTA FLUVIATILIS. 183 
The inner Jamina 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 lamine 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 laminee 
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 filled with young are very 
much distended, the thickness being several millimetres. The form of 
the young shells 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 (Pl. 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 (Pl. 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. 
MUSCULAR 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.), issituated near the anterior mar- 
