h_, collar; _k_, gill-slits; _d_, digestive posterior
intestine; _v_, intestinal vessel; _a_, anus.]

[Illustration: FIG. 49.--A large Sea-lamprey (_Petromyzon marinus_),
much reduced in size. (After Cuvier and Haeckel.) A series of seven
gill-slits are visible.]

[Illustration: FIG. 50--Adult Shark (_Carcharias melanopterus_).
(After Cuvier and Haeckel.)]

[Illustration: FIG. 51.--Diagram of heart and gill-arches of a fish.
(After Owen.)]

[Illustration: FIG. 52.--One gill-arch, with branchial fringe
attached. (After Owen.) H, Heart.]

[Illustration: FIG. 53.--Diagram of heart and gill-arches in a
lizard. (After Owen.) The gill-arches, _a a' a''_, and _b b' b''_,
are called aortic arches in air-breathing vertebrata.]

Well, I have just said that in all the gill-breathing Vertebrata, this
mechanism of gill-slits and vascular gill-arches in the front part of
the intestinal tract is permanent. But in the air-breathing Vertebrata
such an arrangement would obviously be of no use. Consequently, the
gill-slits in the sides of the neck (see Figs. 16 and 57, 58), and the
gill-arches of the large blood-vessels (Figs. 54, 55, 56), are here
exhibited only as transitory phases of development. But as such they
occur in all air-breathing Vertebrata. And, as if to make the homologies
as striking as possible, at the time when the gill-slits and the
gill-arches are developed in the embryonic young of air-breathing
Vertebrata, the heart is constructed upon the fish-like type. That is to
say, it is placed far forwards, and, from having been a simple tube as
in Worms, is now divided into two chambers, as in Fish. Later on it
becomes progressively pushed further back between the developing lungs,
while it progressively acquires the three cavities distinctive of
Amphibia, and finally the four cavities belonging only to the complete
double circulation of Birds and Mammals. Moreover, it has now been
satisfactorily shown that the lungs of air-breathing Vertebrata, which
are thus destined to supersede the function of gills, are themselves the
modified swim-bladder or float, which belongs to Fish. Consequently, all
these progressive modifications in the important organs of circulation
and respiration in the air-breathing Vertebrata, together make up as
complete a history of their aquatic pedigree as it would be possible for
the most exacting critic to require.

[Illustration: FIG. 54