1 68 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 Ammoco?tes, 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 



