THE EVIDENCE OF THE RESPIRATORY APPARATUS 165 
segment, and does not supply any portion of the neighbouring branchial 
segments. The nerve-supply in Ammoccetes gives no countenance to 
the view that the original unit was a branchial pouch, the two sides 
of which each nerve supplied, but is strong evidence that the original 
unit was a branchial appendage, which was supplied by a single 
nerve with both motor and sensory fibres. 
Any observer having before him only this picture of the respiratory 
chamber of Ammoccetes, upon which to base his view of a vertebrate 
respiratory chamber, would naturally look upon the branchial unit of 
a vertebrate as a gilled appendage projecting into the open cavity of 
the anterior part of the alimentary canal or pharynx. This is not, 
however, the usual conception. The branchial unit is ordinarily 
described as a gill-pouch, which possesses two openings or slits, an 
internal one into the lumen of the alimentary canal, and an external 
one into the surrounding medium. This view is based upon embryo- 
logical evidence of the following character :— 
The alimentary canal of all vertebrates forms a tube stretching 
the whole length of the animal; the anterior part of this tube 
becomes pouched on each side at regular intervals, and the walls of 
each pouch becoming folded form the respiratory surfaces or gills. 
The openings of these separate pouches into the central lumen of the 
gut form the internal gill-pouch openings; the other extremity of 
--the pouch approaches the external surface of the animal, and finally 
breaks through to form a series of external gill-pouch openings. 
From the mesoblastic tissue, between each  gill-pouch, there is 
formed a supporting cartilaginous bar, to which are attached a system 
of branchial muscles, with their nerves and blood-vessels. These 
cartilaginous bars, in all fishes above the Cyclostomata, form a 
supporting framework for the internal gill-slit, so that the gills 
are situated externally to them ; the more primitive arrangement is, 
as already mentioned, a system of cartilaginous bars, extra-branchial 
in position, so that the gills are situated internally to them. 
From this description of the mode of formation of the respiratory 
apparatus in water-breathing vertebrates the conception has arisen 
of the gill-pouch as the branchial unit, a conception which is 
absolutely removed from all idea of a branchial unit such as is 
found in an arthropod, viz. an appendage. 
This conception of spaces as units pervades the whole of embryo- 
logy, and is the outcome of the gastrula theory—a theory which 
