90 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
promising method of approaching the subject would seem to be through 
the simpler forms. Some of the homodynamously segmented animals 
offer advantages in this respect which no other forms do. 
The following considerations may be noted as applying particularly 
to the polychaete annelid, Nereis, as a favorable subject for this kind of 
study :—(1) Nereis has a sharply centralized nervous system, consisting 
of well defined ganglia, which occupy very definite relations to the parts 
they control. (2) The histological elements of the nervous system are 
highly differentiated and constant in their relationships. (3) A typical 
body segment of the animal is simple in structure, having a compara- 
tively small number of muscles. (4) There is almost no serial differen- 
tiation of the body segments, excepting in a few of the anterior and the 
anal metameres ; hence it is necessary to determine the structure of one 
segment only in order to know the structure of nearly the whole animal. 
(5) Since there are only a few muscles, the movements of the animal are 
limited in number, and may readily be analyzed and classified. (6) Phy- 
siological experiments may be performed with more than usual facility, 
because the worms are hardy and live well in the aquarium. To these 
considerations may be added the eminently practical one that Nereis 
may be easily obtained in unlimited quantities. With such material 
the problem before us seems to be presented in a comparatively simple 
form. By these considerations I was led to take up the study of the 
nervous system of Nereis. Some of the results obtained are given in 
what follows. 
Methods. 
The material used for intra-vitam staining was the atokal form of 
Nereis virens Sars. It was obtained from the muddy banks of the 
Charles River at a place where the water at low tide contains less than 
0.3% salt. The worms found here may be transferred to fresh water 
without suffering serious injury. For ordinary histological preparations 
both atokal and epitokal forms were used. The latter were obtained 
from the mud flats of Lynn Harbor at the mouth of the Saugus River. 
Before killing, the specimens were narcotized with chloral hydrate or 
alcohol, and the intestine cleaned by forcing a stream of water through 
it. The body was always opened to allow rapid penetration, and some- 
times the intestine was removed, Two methods of fixing and staining 
were employed: either fixing in corrosive sublimate and staining in iron- 
hamatoxylin, or fixing and staining by osmic acid. The value of corro- 
sive sublimate as a fixing agent is well known, and I obtained excellent 
