252 THE OLD MOUTH AND THE NEW. 



the projecting parts of leg-jaws, as in many insects and arachnids; or microscopic 

 organisms must be kicked toward, or into the mouth, by the movements of the 

 legs, or of the posterior part of the body, as in cirripeds and cladocera. 



In some cases, a combination of such conditions has actually led, in the later 

 stages of development, to the complete closure of the foregut, as in the adults 

 of certain lepidoptera and ephemeridae, which cease to feed after metamorphosis; 

 or in certain cirripeds, where the stomodaeal opening into the midgut is closed 

 soon after the larval stages. (Figs. 280, 281.) 



Thus we may recognize in the arthropods a steady, underlying trend to- 

 ward a more compact, voluminous nervous system, and toward a less efficient 

 stomodaeum. These internal conditions rigidly prescribe the possible modes of 

 life that are open to the animals in which they prevail. The pending extinction of 

 the foregut is the dominant factor in the life history of the arachnids, for it 

 has made a liquid diet, sucked through capillary tubes, imperative. For that 

 reason a blood sucking, or a parasitic mode of life, is practically universal among 

 them, just as sucking the blood of animals, or the juices of plants, is universal 

 in certain groups of insects. In all these cases, the animals appear to be making the 

 best of a desperate situation, adjusting their lives with great precision to meet the 

 inevitable march of events within. 



Another important factor in the closing of the old mouth was the conversion 

 of the medullary plate into a medullary tube. This process is well advanced in 

 the arthropods, reaching its highest development in the arachnids. There a 

 true cerebral vesicle is formed that includes the whole forebrain, although the 

 advancing palial fold of the embryo just fails to reach and enclose the oral region. 

 (Fig. 46, B.) 



In the arachnids we may recognize all the important preliminary steps, such 

 as the formation of neural crests, axial infolding, and a forebrain palium, lead- 

 ing up to the conversion of the medullary plate into a medullary tube. But in no 

 arthropod does the process reach a condition that definitely shuts up the oral 

 opening inside the neural tube, thereby cutting off access to the foregut from the 

 outside world. 



But this event does take place in the vertebrates. In the frog embryos, 

 during the open medullary plate stage, we may see a minute pit with a faint 

 prominence in front of it that appears to represent the rostrum and stomodaeal 

 infolding of the arachnids. (Fig. 25.) As the medullary plate closes, the pit 

 deepens, giving rise to the infundibulum, or the ancient passageway for the 

 stomodaeum, while the epithelial sac that lines it and projects out of it, is the 

 saccus vasculosus and the posterior part of the hypophysis, or the remnants 

 of the stomodaeum itself. 



We have shown in the chapters on the nervous system that the location and 

 relations of the principal nerve centers and tracts, and especially the location of 

 the primary gustatory and swallowing centers is entirely in harmony^with this 

 view. 



