THE GUT POUCHES. 



267 



In primitive vertebrates, a variable number of lateral gut pouches are formed 

 belonging to the postoral group of metameres. They unite with the adjacent gill 

 pockets, and thus establish a communication between the gut and the exterior, via 

 the enteric diverticula and the ancestral gill pockets. 



171 



172 



FIG. 171, 172. Larva of Polypterus senegalus, showing rudimentary maxillary appendages, or adhesive papillae. 



After Budgett. 



What caused the opening of one organ into the other in ancestral vertebrates 

 we do not know, any more than we know why it actually takes place now in the 

 embryos of modern vertebrates. For our present purpose, it is sufficient to show 

 that in the arachnid ancestors of the vertebrates the gill pockets and enteric diver- 

 ticula stand in the same relation to each other and to the rest of the head that 



FIG. 173. Larvae of Protopterus annectens, showing the highly developed external gills. A, Seventh day em- 



byro; B, tenth day. After Budgett. 



they now do in vertebrate embryos before the perforation takes place. We are 

 here dealing with one of those cases where there are no intermediate stages between 

 two very different conditions, for either the gut tubes open into the gill chamber, 

 or they do not; if they do, we are dealing with animals of the vertebrate type; if 

 not, with those of the invertebrate type. 



