462 SUMMARY AND CONCLUSION. 



volume of the sensory and nervous centers located in the more anterior metameres 

 of the head and trunk, and the corresponding decrease of the lateral plates, to- 

 gether with the alimentary, locomotor, and mesodermic elements belonging to 

 the same metameres. 



Structural changes of very great moment are inevitably brought about by these 

 conditions. Owing partly to the presence of yolk, from which all the growing 

 tissues can draw their sustenance without the intervention of an alimentary canal, 

 the nervous system attains a very considerable volume, and becomes functional 

 long before the stomodaeum is called into action. This delay in its development, 

 its unfortunate location, and its comparatively delicate epithelial walls, places the 

 stomodaeum under a heavy handicap in its competition for space with the nervous 

 system. The result is that the increasing size, precocity, and more intimate union 

 of the cephalic neuromeres during early embryonic development, leads to a gradual 

 narrowing of the nerve ring surrounding the oesophagus, making in many arthro- 

 pods, for a longer or shorter period, a fluid, or semi-fluid diet {i.e., blood-sucking, 

 parasitic, or scavenging) more and more imperative. The same increase in 

 volume and in precocity of the cephalic neuromeres also led at an early embryonic 

 period to the invagination of the entire brain, so that with the closure of the neural 

 crests and palial fold, the old mouth, which opens into the floor of the brain 

 chamber, became completely shut off from the outside world, and could no longer 

 perform its normal functions. 



The increasing precocity of the cephalic neuromeres and the diminishing 

 volume of the corresponding lateral plates also lead to the formation of a more and 

 more prominent head fold, and to the transfer of the oral arches, which in the 

 arthropods are neural or lateral in position, to the haemal surface of the head. 

 (Figs. 32, 33, 157.) There they converge toward the old dorsal organ and 

 cephalic navel that constitutes the center for the formation of the new mouth. 



The actual closure of the old mouth, the opening of the new one, and the 

 transfer of the jaws to the haemal surface of the head, were, therefore, brought 

 about in a large measure by the action of the same forces. These events, by the 

 final upsetting of a long established organic equilibrium, took place rapidly, the 

 consummation of one event probably accelerating the other; they also led to a 

 rapid readjustment in other parts, and to important changes in the mode of life, 

 especially in the mode of feeding and in the position of the body in locomotion. 



F. The Creation of a New Environment for the Eyes. — The location 

 of the vertebrate eye in the walls of a hollow brain has caused much discussion. 

 We have shown that these so-called cerebral eyes did not originate in situ as 

 the result of the stimulating effect of light acting on the brain through the body 

 walls of a transparent ancestor, or by use, or natural selection. They are 

 merely the ancient median and lateral eyes of the arthropods in a new position. 

 They have been forced into the brain chamber by an accident, as it were, because 



