ANATOMY AND PHYSIOLOGY OF ANODONTA FLUVIATILIS. 175 
animal and lining the interior of the shell, extending between the 
rudimentary teeth of the Anodonta and the strong teeth of the Unio 
and Margaritana. This membrane is called the pallium or mantle. 
The mantle is divided into two lobes, each one lining a single valve of 
the shell; they are united along the dorsal portion, and free along 
the ventral portion, and are pierced by the anterior and posterior 
adductor muscles. 
The mantle is attached to the shell by small muscles near the umbones 
or beaks and along the pallial line. 
The impressions left by the umbonal muscles on the shell are com- 
paratively faint; those left by the muscles of the pallial line are fre- 
quently quite distinct. ‘The edges of the mantle meet on the upper por- 
tion of the anterior adductor muscle, and shortly blend, showing the 
line of junction by a narrow, slightly elevated ridge, which continues 
for nearly one-half the distance to the posterior muscle, then becoming 
stronger, more elevated, gradually increasing in height and width to 
a point a little anterior to the posterior adductor muscle. At this 
point the edges become free, though the lobes are united below; the 
edges at first lie close together, but gradually become more separated, 
continuing thus to the posterior part of the shell, where they are 
entirely free. 
The ridge on the dorsal portion of the united mantle is caused by ~ 
the insertion of the mantle between the rudimentary teeth. Behind 
the body of the animal the outer lamelle of the gills are attached to 
the mantle lobes, the inner lamell to the outer lamellx of the inner 
gills, the inner lamellz of the inner gills are attached to each other, 
thus forming a connection between the lobes, and dividing the space 
between them into two parts or cavities. The inferior and largest cavity 
is known as the branchial or pallial chamber ; the superior is known as 
the anal or cloacalchamber. At the posterior portion of the branchial 
and cloacal chambers are situated, respectively,the rudimentary ventral 
or inhalent siphon (Pl. 5, i.s.) and the dorsal or exhalent siphon 
(Pl. 5, e. s.) formed by the thickened portion of the mantle, just within 
the edge, which is capable of considerable expansion and contraction, 
When the animal is undisturbed the shell is generally slightly opened, 
the siphons expanded and projecting beyond the edges of the valves 
(Pl. 3, i.s.). The portion of the mantle representing the inhalent siphon 
has numerous tentacles (Pl. 3,1.s.; 4, fig. 9), which are developed from 
the inner portion of the mantle. On the exhalent siphon they exist only 
in a rudimental state, though in some species of Unio the tentacles 
on the exhalent siphon are of nearly or quite the same strength as the 
tentacles of the inhalent siphon. The tentacles of the inhalent siphon, 
in a moderate sized specimen of this species, are about two mm.in length 
