THE ORAL ARCHES. 255 



I have called it a cephalic navel, or cephalic fistula, because it is a center of con- 

 vergent growth; a region of ingrowth, or outgrowth, around the point where the 

 haemal surface of the head and the gut is finally closed. 



The cephalic navel usually closes during the embryonic stages, leaving, in a 

 morphological sense, a barren area behind. But in many cases, either the dorsal 

 organ, or some other organ in the same place, persists as an adhesive disc, or a 

 voluminous outgrowth, by means of which the animal attaches itself to some for- 

 eign object. In some parasitic cirripeds, the cephalic outgrowth is buried in the 

 tissues of the host, the old mouth .closes, and the animals are then said to absorb 

 nutrition through the walls of the cephalic outgrowth. 



We have merely to assume for certain stages a somewhat longer duration than 

 now occurs in any arthropod, to make the dorsal organ a new gateway to the gut; 

 for surely a cephalic fistula leading into the midgut, permanently open at both 

 ends, and used to hold fast to animate, or to inanimate objects, is competent to 

 take the place of the old mouth. 



The closing of the old mouth and the evolution of the new one, therefore, was 

 going on in the same animals at the same time. The critical period of substituting 

 one for the other was during the embryonic stages, when both of the organs may 

 have opened into the gut at the same time. During the embryonic stages, a con- 

 siderable time would be available for readjustment, for owing to the large amount 

 of food yolk in the eggs of primitive vertebrates, a relatively long time might 

 elapse before the absorption of food from without became imperative. Whether 

 the actual closing of the old mouth, or the opening of the new one, took place first 

 or last, is of little consequence, for the consummation of one event would probably 

 accelerate the advent of the other. 



III. THE JAWS OR ORAL ARCHES. 



In reconstructing the history of the vertebrate mouth, it is not enough merely 

 to account for the closing of the old mouth and the origin of the new opening into 

 the alimentary canal. To make the account complete, it is necessary to explain 

 the origin of the important organs which surround it, such as the jaws, or oral 

 arches, the hypophysis, tear duct, and the principal outgrowths from the adjacent 

 pharyngeal cavity. 



The circumoral organs of primitive vertebrates are best interpreted as the 

 remnants of several pairs of arthropod leg-jaws, or other appendages, that have 

 been crowded onto the haemal surface by the peculiar mechanical conditions which 

 prevail in the developing head. 



An arthropod appendage may be defined as an ectodermic outgrowth consist- 

 ing of several branches, or stems, located on the haemal or lateral side of the body, 

 adjacent to the main nerve axis. In the higher forms, e.g., the arachnids, one stem 

 forms the typical appendage. On its median basal margin is a prominent sensory 

 spur, or gustatory organ, from which in the early embryonic stages arises a large 



