286 JOURNAL OF COMPARATIVE NEUROLOGY. 
thus scantily represented the individual rates show only slight 
variation, as in Arenicola (table IV, page 270), Aphrodite (table 
VI, page 273), Polynoe and Sthenelais (tables VII and VIII, 
pages 274 and 275), and Nereis virens (table XI, page 280), and 
their representative means may therefore not be far from the 
true rate in these species. 
The rate of the nervous impulse in this series of worms is 
as varied as the structure and the habit of the worms them- 
selves, the lowest being in Cerebratulus with a rate of only 5 to 
g centimeters per second and the highest in Bispira with a rate 
of 7 meters persecond. But there seem also to be differences 
in rate between species in which little difference in structure 
and habits exists, as in the cases of Eunice and Nereis (tables 
IX, X and XI), the former showing a rate of 466 cm. per sec- 
ond, the latter a rate of 165 cm. per second. 
In the species which permitted of determining the rate of 
the impulse in both directions of the cord (Nereis, table XI; 
Lumbriconereis, table XIII, and Glycera, table XIV) there 
seems to be no constant difference between the antero-posterior 
and the postero-anterior rates. : 
A question of considerable interest is whether the rate in 
the ventral nerve cord as determined in the present work repre- 
sents the rate through continuous nerve fibers as in a verte- 
brate muscle-nerve preparation or whether the nervous paths. 
are more complex. Our knowledge of the structure of the 
ventral nerve cord of the worms has been greatly advanced 
within the last few years through the researches of RETZIUs. 
(10, 1891, 1892, 1894, 1898, 1g00), Ruope (11, 1891), Brep- 
ERMANN (12, 1891), LENHOSSEK (13, 1892), FREIDLANDER (14, 
1894), ApAruy (15, 1897) and Haver (16, 1899). But while 
many of the histological elements have thus been worked out and 
found to be markedly uniform throughout the phylum, it seems 
that the number of segments with which a motor neuron comes. 
in direct relation and the extent and relation of the small sen- 
sory fibers in the longitudinal tracts and of the central or ‘‘as- 
sociation cells’’ are yet undetermined. Even the applicability 
of the neuron conception to the nervous system of the worms 
