Embryology. 147 
arrangement is permanent. It is likewise met with in a 
peculiar kind of worm, called Balanoglossus—a creature 
so peculiar, indeed, that it has been constituted by 
Gegenbaur a c/ass all by itself. We can see by the 
wood-cuts that it presents a series of gill-slits, like the 
homologous parts of the fishes with which it is compared 
—i.e. fishes ofa comparatively low type of organization, 
which dates from a time before the development of 
external gills. (Figs. 48, 49, 50.) Now, as I have 
already said, these gill-s//ts are supported internally by 
the gill-arches, or the blood-vessels which convey the 
blood to be oxygenized in the branchial apparatus 
(see below, Figs. 51, 52, 53);. and the whole arrange- 
ment is developed from the anterior part of the in- 
testine—as is likewise the respiratory mechanism 
of all the gill-breathing Vertebrata. That so close 
a parallel to this peculiar mechanism should be met 
with in a worm, is a strong additional piece of evidence 
pointing to the derivation of the Vertebrata from the 
Vermes. 
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. Con- 
sequently, 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 
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