./ 



l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



NC), and it seems reasonable to attribute the special development of 

 these nerves in a creeping animal to the special development of sensi- 

 tivity in the ectoderm along the edges of the body in contact with the 

 supporting surface. 



That the nerve strands were primarily unsegmented is shown by the 

 fact that they are not ganglionated in the Archiannelida, and by their 

 condition in Pcripatiis where the nerve cells are still distributed along 

 the length of the cords, and segmental grouping of the cells is but 

 slight. A concentration of the nerve cells of the cords in each seg- 

 ment is, then, only a simple adaptation to efficiency where metamerism 

 l)ecomes the established body structure. After the segregation of the 

 nerve cells into pairs of segmental ganglia, the intervening fibrous 

 tracts of the cords remain as connectives between the successive ganglia 

 in each chain, while transverse ventral nerves, originally going from 

 one cord to the other, become commissures uniting the ganglia of 

 each segmental pair. In this way, apparently, a simple nervous system, 

 formed primarily as two parallel strands of nerve tissue, became a 

 segmented system of the ladder type (fig. 12 B) . In the further course 

 of evolution, the ganglia of each segment come together medially and 

 combine into a single ganglionic mass, or segmental ganglion (C), 

 which, in some arthropod groups, acquires an addition from a sec- 

 ondary median cord of nerve tissue developed from the ventral 

 ectoderm along the midline of the body. The transverse commissures 

 are now internal fibrous tracts of each double ganglion, but the length- 

 wise cords persist usually as paired interganglionic connectives. Each 

 definite body ganglion, or pair of ganglia, innervates, in general, only 

 the parts and organs of its own segment, but all the ganglia show a 

 tendency to migrate along the cords, especially in a cephalic direction, 

 and to unite with other ganglia to form composite ganglionic masses. 

 Whatever may be the final position of any pair of ganglia, however, 

 its nerves in most cases still go to the segment in which the ganglia 

 originated. The nervous system, thus, often gives a key to the body 

 segmentation where the latter is obscure or obliterated. 



The next important stage of development is that, characteristic of the 

 arthropods, in which are formed the external segmental appendages. 

 The organs designated " appendages " in the limited sense are hollow, 

 ventro-lateral outgrowths of the body wall (figs. 13, 14, 22), which 

 become movable by muscles inserted on their bases, and flexible by a 

 series of joints in their walls, also provided with muscles. Here again, 

 we connect structural evolution with movement, for undoubtedly the 

 segmental appendages in the first place were all organs of locomotion, 

 giving a new power of movement supplanting the wriggling and 



