250 THE OLD MOUTH AND THE NEW. 



pouches that lead from the gut to the exterior; and the whole or a part of the 

 respiratory tissue of the gill pouch is said to arise from the endoderm. 



These striking differences are apparently irreconcilable, and have led many 

 zoologists to the conclusion that there can be no direct genetic relation between 

 these two groups of animals. We shall show in this chapter that there is no real 

 foundation for this belief, for when the facts are known and their meaning is 

 made clear, it will be seen that the vertebrate organs in question are of the same 

 nature as those in arthropods. It is true they have undergone a remarkable meta- 

 morphosis, but it is one brought about in a perfectly natural and consecutive 

 manner by the action of definite internal forces that can be recognized and their 

 probable effects, in a measure, estimated. 



Argument. — Briefly stated, the argument and the evidence to be presented is 

 as follows : We shall show i, that during the evolution of the arthropods the primi- 

 tive entrance to the midgut was being gradually closed, and in some cases actually 

 was closed, because the mouth was shifting into a more and more inaccessible 

 position, and because the stomodaeum was becoming more and more constricted 

 by the growth of the surrounding organs. The remnants of this now useless 

 passageway may still be seen in its proper position on the floor of the closing 

 neural canal of vertebrates; this passageway is the infundibulum, and the remnants 

 of the foregut is the saccus vasculosus and the posterior part of the hypophysis. 



2. That the foundations of a new mouth are already established in arthropods 

 in the cephalic navel, or so called "dorsal organ," which lies on the haemal side 

 of the head in a position corresponding to that of the mouth in vertebrates. It 

 affords a transitory opening from the exterior into the midgut, and it, or the adja- 

 cent tissues, may serve as a means of attaching the animal to foreign objects, or 

 to its host. Thus in the arthropods, an organ of very great antiquity and habits 

 long established are prepared to perform the work of a new mouth after the verte- 

 brate fashion^ as soon as the old mouth becomes permanently closed. 



3. That in the arthropods several pairs of leg-jaws surround the mouth on the 

 neural surface of the body, and that the prevailing conditions in the arthropod 

 head tend to crowd the basal portions of these appendages toward the haemal sur- 

 face so that they converge around the infolding cephalic navel in the same manner 

 that the oral arches of vertebrates converge around the mouth. It will be shown 

 that in vertebrate embryos the oral arches first appear on the neural surface as 

 three or four pairs of appendicular arches, and that they then gradually shift 

 toward the haemal side, converging toward the "anlage" of the new mouth, and 

 forming the paired rudiments of the premaxillary, maxillary mandibular, and 

 hyoid arches. These paired arches finally unite, the first two pairs forming the 

 upper, and the third pair, the lower jaw. 



4. That in the ostracoderms, the oldest fossil vertebrate-like arthropods, the 

 mouth lies between paired jaws, which in chewing move to and from the middle 



