68 The Swan-Mussel and its Anatomy. 



the animal, is more readily made out than in many other 

 bivalves. 



The shell of the Anodonta cygnea differs from that of the 

 other Unionidce in the absence of distinct teeth in its hinge ; 

 it is oblong in shape, and compressed for the whole space be- 

 tween the hinge and the posterior extremity; in colour it 

 varies much, young specimens being light olive, with dark 

 concentric bands or lines, which are supposed by some to mark 

 the animal's growth ; at the region of the umbo the shell is 

 always pearly from the erosion of the outer skin or epi- 

 dermis ; but what causes the erosion in this particular point 

 it is very difficult to determine ; * older specimens are darker 

 in colour ; the valves of the shell internally are of a whitish 

 or yellowish pearly lustre, and sometimes very iridescent ; the 

 marks of the muscles and mantle, so distinct in some marine 

 Conchifers, are very faint in the Anodonta. 



The animal that tenants the shell is represented in Figure 

 2 ; d representing one of the external pair of gills greatly dis- 

 tended with thousands of young Anodons, which bear no resem- 

 blance whatever to the parent animal. The Anodon is attached 

 to the shell by two muscles, the anterior and posterior adductor ; 

 its parts consist of the mantle, a thin fleshy envelope, which 

 covers the whole body of the animal, and consists of two folds 

 or leaves which are joined at the back part but open in front. M. 

 Moqivin-Tandon well compares the disposition of the two lobes 

 of the mantle to the " covers of a book when it is placed on 

 its edge with the back uppermost." Along the margin of the 

 dorsal region the mantle forms a raised seam, or line, with a 

 narrow slit posteriorly. The mantle secretes the shell; it is 

 much thickened at the extreme margin of its free portion. 



The organization of a fresh-water mussel, like the Con- 

 chifera in general, is simple, though there are some points 

 which it is extremely difficult to determine with anything like 

 precision. The Anodon is provided with organs of digestion, 

 circulation, respiration, locomotion, and generation ; the 

 nervous system also is well developed. 



The organs of digestion consist of mouth, stomach, and in- 

 testine, f The mouth of an Anodon is a horizontal opening 

 situated between the two pairs of labial palpi, under the 

 auterior juncture of the mantle and just above the foot. The 

 labial palpi are beautifully striated transversely, of a triangularly 



* Mr. Jeffreys believes this erosion is caused by the chemical action of gases 

 evolved from the mud in which this portion of the shell is usually imbedded ; but 

 the umbonal region is just that part of the shell which is generally not imbedded. 



t r -lhe food of the swan-mussel consists of Desmidese, Diatomacea 1 , and other 

 minute organisms, such as the spores of Algee ; this vegetable diet is accompanied 

 with Rotifer a, minute Entomostraca, etc. 



