The class of _Fish_ (Pisces) with which we accordingly begin the series of Double-nostriled animals, is distinguished from the other six classes of the series by the swimming bladder never developing into lungs, but acting only as a hydrostatic apparatus. Agreeing with this, we find that in fish the nose is formed by two blind holes in front of the mouth, which never pierce the palate so as to open into the cavity of the mouth. In the other six classes of double-nostriled animals, both nostrils are changed into air passages which pierce the palate, and thus conduct air to the lungs. Genuine fish (after the exclusion of the Dipneusta) are accordingly the only double-nostriled animals which exclusively breathe through gills and never through lungs. In accordance with this, they all live in water, and both pairs of their legs have retained the original form of paddling fins. SYSTEMATIC SURVEY _Of the 7 Legions and 15 Orders of the Fishes._ --------------------------------------------------------------------------------- _Sub-classes_ | _Legions_ | _Orders_ | _Examples_ _of_ | _of_ | _of_ | _from_ _Fishes._ | _Fishes._ | _Fishes._ | _the Orders._ --------------------------------------------------------------------------------- { { { 1. Sharks Sharks, dog-fish { I. Transverse { _Squalacei_ A. { mouths { 2. Rays Spiked rays, electric =Primæval= { _Plagiostomi_ { _Rajacei_ rays, etc. =Fish= { +Selachii+ { II. Sea-Cats { 3. Sea-Cats Chimæra, Calorrhynchias { _Holocephali_ { _Chimæracei_ { { 4. Buckler-heads Cephalaspidæ, Placoderma, { III. Mailed Ganoid { _Pamphracte_ etc. { Fish { 5. Sturgeons Spoon-sturgeons, sturgeons, { _Tabuliferi_ { _Sturiones_ sterlet, etc. { { { 6. _Efulcri_ Double-finned B. { IV. Angular-scaled { 7. _Fulcrati_ Palæoniscus, bony pike, =Ganoid= { Ganoid Fish { etc. =Fish= { _Rhombiferi_ { 8. _Semæopteri_ African finny pike, etc. +Ganoides+ { { V. Round-scaled { 9. _Cœloscolopes_ Holoptychius, Cœlacanthides, { Ganoid Fish { etc. { _Cycliferi_ { 10. _Pycnoscolopes_ Coccolepida, Amiadæ, etc.