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CONCLUSION

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EVOLUTIONARY STAGES OF THE PRINCIPAL GROUPS OF VERTEBRATA^[1]

STAGES OF THE PAIRED LIMBS.		CLASSES.		STAGES OF THE HEART.
I. Adactylia s. Impinnata. Without jaws and limbs.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	1. Acrania.		I. Leptocardia. Cold-blooded; heart with one chamber; without lungs.
		2. Cyclostomata.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	II. Ichthyocardia. Cold-blooded; heart two-chambered, with one atrium and one ventricle; heart containing venous blood only; without lungs.
II. Polydactylia s. Pinnata. With two pairs of fins.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	3. Pisces.
		4. Dipnoi.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \\\ \\\ \ \end{matrix}}\right\}\,}$	III. Amphicardia. Cold-blooded; heart with three complete chambers, namely, with two atria and one ventricle, or (Reptilia) two ventricles with still incomplete septum; heart containing mixed venous and arterialized blood; with lungs.
III. Pentadactylia s. Tetrapoda. With two-pairs of pentadactyle limbs (unless they have been lost by reduction).	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	5. Amphibia.
		6. Reptilia.
		7. Aves.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	IV. Thermocardia. Warm-blooded; heart with four complete chambers, namely, two auricles and two ventricles; right half of the heart with venous, left half with arterialized, blood; with lungs.
		8. Mammalia.