128 DESCRIPTIONS OF PREP A RA TIONS, 



Formation of Pearls. Pagenstecher, Z. W. Z. ix. 1858, von Hessling, Die 

 Perlmuscheln und ihre Perlen, Leipzig 1859, and in Z. W. Z. ix. 1858, p. 453. 



Composition of she!! in relation to lime in water, &c. Voit, Z. W. Z. x. 1859-60. 

 Vegetable parasites in shell, &c. Kolliker, Z. W. Z. x. 1859-60. 



25. FRESH-WATER MUSSEL (Anodonta cygnea), 



Removed from its shell and suspended so as to show the general external features 



of the animal. 



THE animal is suspended by the anterior adductor muscle, and the two 

 folds (right and left) of the mantle have been turned back and fastened 

 over the dorsal area. The mantle-folds are free throughout their whole 

 extent, but they are united indirectly at the posterior end behind the 

 foot by the attachment of the branchiae. The mantle cavity is in this 

 way divided into two chambers, an inferior large infra-branchial chamber, 

 and a superior small supra-branchial chamber. In the living animal the 

 ventral edges of the mantle-folds are applied more or less closely to one 

 another, even when the foot is protruded, and water finds its entrance 

 to the infra-branchial chamber through the inferior siphonal notch. The 

 edges of this notch are covered with tentacles, and hence the portion 

 of the mantle in question is readily recognised. It is posterior, and lies 

 immediately below the attachment of the branchiae. The supra-branchial 

 chamber opens above the attachment of the branchiae by the superior 

 siphonal notch, which has smooth non-tentaculate edges. A white bristle 

 is passed into it in this specimen. The two notches in many Lamelli- 

 branchiata are prolonged, the inferior into the inhalent, the superior into 

 the exhalent, siphon. The two chambers, however, in the Anodon com- 

 municate one with the other, not only indirectly through the cavities 

 of the branchiae, but directly also along the posterior part of the base 

 of the visceral mass, where, as may be seen in this specimen, the innermost 

 branchial lamella has a free edge. In some Lamellibranchiata with an 

 aborted foot this gap does not exist, and consequently the two chambers 

 do not communicate directly. 



The free edges of the mantle folds are thickened, and correspond to 

 the collar in the Snail. Two main lips, best developed posteriorly, 

 run along the free edge, and they inclose two somewhat smaller ridges 

 not always discernible. The outermost ridge is prolonged round the 

 superior siphonal notch, and unites at some distance from it on the dorsal 

 surface with its fellow to form the dorsal ridge or raphe. 



The foot projects in the middle line. It is continuous with the visceral 

 mass, which contains coils of the digestive canal, liver (=hepato-pancreas), 

 and generative organs, and is much dilated. The foot proper is purely 

 muscular, and may be distinguished by its yellow tint and comparative 



