4 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
peripheral nervous system of the Copepoda, although the literature 
which deals with the group mainly from a systematic standpoint may 
contain scattered statements concerning the nervous system and 
sense organs. Even the little work that has been done deals almost 
exclusively with the gross anatomy of the nervous system. Such 
matters as the fibre tracts and systems of neurons have received the 
most fragmentary and superficial treatment. Work of this kind must 
now be regarded as relatively unsatisfactory because of the advance 
in general technique and neurological methods during the last few 
years. A thorough, painstaking study of any one of the Copepoda 
is worth while because of the comparatively low organization of the 
group, both for the sake of comparison with the higher forms of 
Crustacea and for a knowledge of the group itself. 
Within the group of Copepoda the marine forms have been least 
studied, for the paper of Richard (91), which is the most thorough of 
any dealing with the more minute anatomy of the nervous system, 
is concerned with only fresh-water forms. For twenty-eight years 
previous to the appearance of Richard’s work there was none dealing 
primarily and specifically with the nervous system. Claus (’63) 
devoted a portion of his monograph on the non-parasitic Copepoda 
to the anatomy of the marine representatives, and in it we have the 
most complete, as well as the best, comparative study of the nervous 
system and sense organs of the pelagic forms that has appeared. 
The general disregard of the nervous system in the marine Cope- 
poda would alone be reason enough for taking up the study at the 
present time. But in view of the closer attention which the nervous 
apparatus in decapods has received recently, a study of the lower 
forms for purposes of comparison seems especially desirable. ‘The 
present investigation grew out of a desire to gain some knowledge of 
the peripheral nervous system and sense organs of the Copepoda as a 
preliminary step toward a study of the reactions, including the diurnal 
and seasonal movements, of these enormously abundant and very 
important plankton organisms. It is hoped that this paper may be 
one of several dealing with the nervous system, the central organs 
being taken up at a subsequent time. 
For this purpose one of the very commonest calanoid copepods 
found in the waters of the Pacific Coast was selected as a basis for 
study. This form is Eucalanus elongatus Dana. It is particularly 
suitable for work in which microscopical methods are to be employed, 
because it is of large size, and exceedingly transparent; moreover the 
chitin is so thin and delicate that it does not cause the least trouble in 
