THE ORAL ARCHES. 257 



In dealing with the morphology of a true segmental appendage, we must 

 recognize and account for these various parts. 



As a rule, the free part of the oral appendages in arthropods is very small, 

 or even absent, while the basal portion, the somite, sense organ, and ganglion, 

 may be of considerable size. If this condition is associated, as it usually is in the 

 higher arthropods, with a voluminous and precocious forebrain, a large yolk 

 sphere, and with the absence of lateral plate structures, the tendency during the 

 early embryonic periods will be to raise the forebrain off the yolk surface and 

 thrust it forward, leaving the way open for the basal lobes of the more anterior 

 appendages to unite on the hasmal surface, around the center formed by the cephalic 

 navel. (Figs. 17, 31 



FIG. 158. Mercator projections of vertebrate embryos. 



Under the influence of these conditions, the more anterior appendages in 

 certain adult arthropods have been transferred, either to the anterior surface of 

 the head, to a position halfway between the neural and haemal surfaces, i.e., the 

 cheliceras of arachnids (Figs. 17, 43), or almost to the haemal surface, i.e., the 

 antennae of cladocera (Fig. 9), and many parasitic copepods (Figs. 282, 283); or 

 they are transferred definitely to the haemal surface, close to the region of the 

 cephalic navel, i.e., the antennae in all cirripeds. (Figs. 274, 280.) 



It is seen, therefore, that in the arthropods the increasing size and precocity 

 of the forebrain, the degeneration of the lateral members of the anterior meta- 

 meres, the increasing size of the yolk sphere, and the time factors involved in 

 apical growth on a spherical surface, all conspire to crowd the appendages and 

 their associated parts toward the haemal surface of the head. (Compare the 

 mercator projections in Fig. 157 with Figs. 17, 31, 32, 158, 160.) 



Development of the Oral Arches in the Frog. In the embryos of primi- 

 tive vertebrates, the transfer of oral arches to the haemal surface of the head is 

 17 



