neys, and, in short, of most of the organs which belong to the other
Vertebrata. It presents, however, a rudimentary backbone, in the form of
what is called a notochord. Now a primitive dorsal axis of this kind
occurs at a very early period of embryonic life in all vertebrated
animals; but, with the exception of _Amphioxus_, in all other existing
Vertebrata this structure is not itself destined to become the permanent
or bony vertebral column. On the contrary, it gives way to, or is
replaced by, this permanent bony structure at a later stage of
development. Consequently, it is very suggestive that so distinctively
embryonic a structure as this temporary cartilaginous axis of all the
other known Vertebrata should be found actually persisting to the
present day as the permanent axis of _Amphioxus_. In many other
respects, likewise, the early embryonic history of other Vertebrata
refers us to the permanent condition of _Amphioxus_. In particular, we
must notice that the wall of the neck is always perforated by what in
_Amphioxus_ are the gill-openings, and that the blood-vessels as they
proceed from the heart are always distributed in the form of what are
called gill-arches, adapted to convey the blood round or through the
gills for the purpose of aeration. In all existing fish and other
gill-breathing Vertebrata, this 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 class 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 of a 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-_slits_ 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 arrangement is
developed from the anterior part of the intestine--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.

[Illustration: FIG. 48.--_Balanoglossus_. (After A. Agassiz.) _r_,
proboscis; _