TIE INTELLECTUAL OBSERVER. 



SEPTEMBER, 1864. 



THE SWAN-MUSSEL AND ITS ANATOMY. 



BY THE EEV. W. HOUGHTON, M.A., P.L.S. 

 (With a Tinted, Plate.) 



What collector of shells is unacquainted with the form of 

 the fresh-water mussel, as it lies half buried in the mud, 

 with its back or umbonal region, easily recognized by its 

 white pearly appearance, above the surface; or as the same 

 animal lies flat on the bottom of the river or pond, with its 

 large, broad, and semi-transparent foot protruded between the 

 valves of the shell two inches or more ? The swan-mussel 

 (Anodonta), of which the most recent English authority* recog- 

 nizes two British species, the Anodonta cygnea and the Ano- 

 donta anatina, is one of the largest of our bivalve molluscs, 

 and its study a matter of curious and more than ordinary 

 interest, especially in that portion of its history which relates 

 to the development of the young fry. 



The Anodonta,f so named from the absence or very rudi- 

 mentary condition of the teeth of the hinge, belongs to the 

 family Unionidce, a group of the Oonchifera, or Bivalves, of the 

 order Lamellibranchiata, the individuals of which are charac- 

 terized by the possession of four branchiae or gills arranged on 

 each side of the body in pairs. The Lamellibranchiata are 

 all aquatic animals, and the greater part inhabit the sea. Of 

 the fresh- water kinds, the Anodonta and JJnio, the common 

 round or semi-oval Gyclas comeum, and the curious sub-trian- 

 gular shell of Dreissena polymorpha, afford instances of the 

 three native families, the TJnionidoe, Sphairiidai, and Dreis- 

 senidai. 



Let us now glance at some of the most important parts of 

 the anatomy of the swan-mussel, which, from the large size of 



* Gwjn Jeffreys' British Conchology, vol. i. 

 t From a, " not," and oSovs, gen, bUvt os, " A tooth." 

 VOL. VI. — NO. II. E 



