STAGES OF THE CLASSES. STAGES OF THE HEART.
PAIRED LIMBS.
{ 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.