168 THE ORIGIN OF VERTEBRATES 
the segmental ccelomic cavities extend into the limbs. These 
cavities both in the vertebrate and in the arthropod disappear 
before the adult condition is reached. 
The whole evidence thus points strongly to the conclusion that the 
true branchial segmental units are the branchial bars or diaphragms, 
not the pouches between them. 
It is possible to understand why such prominence has been 
given to the conception of the branchial unit as a gill-pouch rather 
than as a gill-appendage, when the extraordinary change of appear- 
ance in the respiratory chamber of the lamprey which occurs at 
transformation, is taken into consideration. This change is of a 
very far-reaching character, and consists essentially of the formation 
of a new alimentary canal in this region, whereby the pharyngeal 
chamber of Ammoccetes is cut off posteriorly from the alimentary 
canal, and is confined entirely to respiratory purposes, its original 
lumen now forming a tube called the bronchus, which opens into the 
mouth and into a series of branchial pouches. 
In Fig. 68 I give diagrammatic illustrations taken from Nestler’s 
paper to show the striking change which takes place at transforma- 
tion, (A) representing three branchial segments of Ammoccetes, and (B). 
the corresponding three segments of Petromyzon. The corresponding 
parts in the two diagrams are shown by the cartilages (br. cart.), the 
sense-organs (S), and the branchial veins (V. br.); the corresponding 
diaphragms are marked by the figures 1, 2, 3 respectively. As is 
clearly seen, it is perfectly possible in the latter case to describe the 
respiratory chamber, as Nestler has done, as divided into a series of 
separate smaller chambers—the gill-pouches—by means of a series 
of diaphragms or branchial bars. The surface of these gill-pouches 
is in part thrown into folds for respiratory purposes, and each gill- 
pouch opens, on the one hand, into the bronchus (Bro.), and, on the 
other, to the exterior by means of the gill-slit. The branchial unit 
in Petromyzon is, therefore, according to Nestler and other mor- 
phologists, the folded opposed surfaces of two contiguous diaphragms, 
and each one of the diaphragms is intersegmental between two gill- 
pouches. 
Nestler then goes on to describe the arrangement in Ammoccetes 
in the same terms, although there is no bronchus or gill-pouch, but 
only an open chamber into which these gill-bearing diaphragms 
project, which open chamber serves both for the passage of food and 
