NO. 6 ANNELIDA, ONYCHOPHORA, AND ARTHROPODA SNODGRASS 57 



correspond with the embryonic somites, except that the first pair 

 (iVO) Hes on the preoral head region. Whether these thickenings 

 represent primitive organs or are merely embryonic structures is 

 open to question, but they suggest the paired tubercles on what may 

 be the ventral surface of Xenusion (fig. 21 C). From the inner 

 surfaces of the ventral organs of the body are differentiated the 



Fig. 24. — Nervous system of the head and of a body segment of Peripatus 

 tholloni Bouvier. (From Fedorow, 1926, 1929.) 



A, diagram of brain and anterior parts of nerve cords, with bases of nerves, 

 dorsal view. B, nerve cord and peripheral nerves of left side of a body segment, 

 lateral view. 



a, sensory antennal nerve; aip, anterior interpedal nerve; A)it, antenna; b, 

 motor nerves of antenna; Br, brain; c, optic nerve; Co)ii, nerve commissure; 

 d, lateral dorsal nerve ; E, eye ; e, nerve to circumoral fold ; f, median dorsal 

 nerve ; g, nerve to dorsal muscles of head ; h, commissural nerve from f to g ; 

 i, stomodaeal nerve; ICom, first ventral commissure; /, k, nerves of jaw; L, 

 leg ; /, nerves of oral papilla ; m, n, 0, nerves of first leg segment ; ip, 2p, first 

 and second pedal nerves ; ipp, 2pp, first and second postpedal nerves ; pip, pos- 

 terior interpedal nerve ; iprp, 2prp, first and second prepedal nerves ; v, ventral 

 nerves. 



ventral nerve cords ; the outer parts are then gradually reduced in 

 size until finally they disappear as distinct areas of the epidermis. 

 When the nerve strands become free cords within the body they do 

 not approach each other or unite as do the nerve cords of most anne- 

 lids or arthropods ; on the contrary they move farther apart until they 

 take positions along the sides of the body on a level with the leg 

 bases (fig. 29, NC^. The definitive cords, moreover, lie laterad of 



