Anatomy and Physiology of x^nodonta 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 lamellae of the gills are attached to 

 the mantle lobes, the inner lamellae to the outer lamellae of the inner 

 gills, the inner lamellae 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 cloacal chamber. At the posterior portion of the branchial 

 andcloacal chambers are situated, respectively, the rudimentary ventral 

 or inhalent siphon (PI. 5, i. s.) and the dorsal or exhalent siphon 

 (PI. 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 (PI. 3,i. 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 



