50 EVOLUTION OF THE NERVOUS SYSTEM IN SEGMENTED ANIMALS. 



is, the central nerve terminals and the centrally located nerve cells, in many cases 

 lie in different metameres from the peripheral organs with which they are asso- 

 ciated. (Figs. 59, 60.) 



This condition appears to prevail in the most primitive arthropod neuro- 

 meres, hence that complete functional and morphological correspondence, sup- 

 posed to occur between a body joint and a nerve cord joint, does not exist. Meta- 

 merism has developed to a different degree in the two systems and affects them 

 in quite a different manner. The morphological segmentation of the nerve 

 axis does not coincide with that of the body, and the functional segmentation of the 

 nerve cord does not coincide with its morphological segmentation, for both motor 

 nerve cells and sensory dendrites are frequently located in a neuromere in front 

 of the metamere in which the corresponding nerve fibers leave the cord, and in 

 which they have their peripheral terminals. 



A partial explanation of the lack of correspondence between functional and 

 morphological metamerism of the nerve cord is afforded by what takes place in 

 embryo scorpions. Here each neuromere is composed of two distinct segments, 

 and as the space between the abdominal ones increases, the anterior segment of 

 one neuromere unites with the posterior segment of the one in front of it, thus 

 completely changing the original grouping of the half neuromeres. It is not 

 clear whether this takes place in Limulus or in the other arthropods I have 

 studied, but it probably does, otherwise it is hard to understand how the cell 

 bodies of the motor neurones are located in the neuromeres in front of the one 

 from which the corresponding motor nerves leave the cord. 



It is therefore clear that Loeb's attempt to prove that each abdominal neuro- 

 mere in Limulus is a complete reflex center for its corresponding gill, is based on a 

 misconception of the structure of the nerve cord. His interpretations of his ex- 

 periments are incorrect because, as we shall show later, they are based on a 

 misunderstanding of the structure of a neuromere and the distribution of its nerves. 



VI. THE PRIMITIVE SENSE BUDS. 



The main nerve trunks in the arthropods represent bands of metamorphosed 

 sense organs, and they coincide with the lines along which such sense organs 

 were distributed in the remote ancestral forms. The transformation of these 

 primitive sense organs into nerve cells constitutes an important step in the evolu- 

 tion of the central nervous system. Many details in this process are still retained 

 in the embryos of arachnids. 



In the scorpion, the entire brain and cord is an aggregate of innumerable, 

 closely packed sense buds which, under a low power, produce a mottled, or pitted 

 appearance that is very characteristic. (Figs. 15 and 16.) Under a higher 

 power, and in sections, each bud appears pear-shaped, with a goblet-shaped cavity 

 opening to the exterior at one end, and leading into a narrow vertical canal at the 

 other. They consist of typical sensory cells, having the same shape, arrangement, 



