{ 1. _Acrania._ I. _Leptocardia._ I. _Adactylia_ { Cold-blooded; heart s. _Impinnata_. { with one chamber; Without jaws { without lungs. and limbs. { { 2. _Cyclostomata._ } II. _Ichthyocardia._ } Cold-blooded; heart } two-chambered, with } one atrium and one } ventricle; heart } containing venous } blood only; without II. _Polydactylia_ { 3. _Pisces._ } lungs. s. _Pinnata_. { With two { } III. _Amphicardia._ pairs of fins. { 4. _Dipnoi._ } Cold-blooded; heart } with three complete } chambers, namely, with } two atria and one } ventricle, or (Reptilia) { 5. _Amphibia._ } two ventricles with still { } incomplete septum; heart { } containing mixed venous { } and arterialized III. _Pentadactylia_ { 6. _Reptilia._ } blood; with lungs. s. _Tetrapoda_. { With two pairs { { IV. _Thermocardia._ of pentadactyle { { Warm-blooded; heart limbs (unless { 7. _Aves._ { with four complete they have { { chambers, namely, two been lost by { { auricles and two reduction). { { ventricles; right half { { of the heart with venous, { { left half with { { arterialized, blood; with { 8. _Mammalia._ { lungs. [24] Abridged from Haeckel's 'Systematische Phylogenie der Vertebraten,' § 14. BIOGRAPHICAL SKETCHES