Mussels, or Plate-gills (Lamellibranchia), possess a bivalved shell like the Lamp-shells. In the latter, one of the two valves covers the back, the other the belly of the animal; whereas in Mussels the two valves lie symmetrically on the right and left side of the body. Most Mussels live in the sea, only a few in fresh water. The class is divided into two sub-classes, Asiphonia and Siphonida, of which the latter were developed at a later period out of the former. Among the Asiphonia are Oysters, mother-of-pearl Shells, and fresh water Mussels; among the Siphonida, which are characterized by a respiratory tube, are the Venus-shells, Razor-shells, and Burrowing Clams. The higher Molluscs seem to have developed at a later period out of those without head and teeth; they are distinguished from the latter by the distinct formation of the head, and more especially by a peculiar kind of tooth apparatus. Their tongue presents a curious plate, armed with a great number of teeth. In our common Vineyard Snail (Helix pomatia) the number of teeth amount to 21,000, and in the large Garden Slug (Limax maximus) to 26,800. SYSTEMATIC SURVEY _Of the 4 Classes, 8 Sub-classes, and 21 Orders of Molluscs._ ------------------+-----------------------+------------------------+------------------- _Classes of_ | _Sub-classes of_ | _Orders of_ | _Systematic Name_ _Molluscs._ | _Molluscs._ | _Molluscs._ | _of the Orders._ ------------------+-----------------------+------------------------+------------------- I. _Molluscs without head or teeth_: ACEPHALA _or_ ANODONTODA. --------------------------------------------------------------------------------------- I. { I. Ecardines { 1. Stalked 1. Lingulida =Lamp-shells= { _Hinge-less_ { 2. Flattened 2. Craniada { +Spirobranchia+ { or { +Brachiopoda+ { II. Testicardines { 3. Fleshy armed 3. Sarcobrachia { _Hinge-less_ { 4. Calcareous-armed 4. Sclerobrachia II. { III. Asiphonia { 5. One-muscled 5. Monomya =Mussels= { _Mussels without_ { 6. Uneven-muscled 6. Heteromya or { _respiratory tubes_ { 7. Even-muscled 7. Isomya =Plate-gills= { { +Lamellibranchia+ { IV. Siphonida { 8. Round-mantled 8. Integripallia or { _Mussels with_ { 9. Ray-mantled 9. Sinupalliata +Phyllobranchia+ { _respiratory tubes_ { 10. Tube-mussels 10. Inclusa --------------------------------------------------------------------------------------- II. _Molluscs with head and teeth_: CEPHALOPHORA _or_ ODONTOPHORA. ---------------------------------------------------------------------------------------