4 bulletin: museum of comparative zoology. 



peripheral nervous system of the Copepoda, ahhough the hterature 

 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 imsatisfactory 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 ajjpearance 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 tlesire 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 ]ilankton organisms. It is hoped that this ])a]ier may be 

 one of several dealing with the nervous svstem, the central organs 

 being taken up at a subsequent time. 



For this j^i^n'pose 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 microsco]iical 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 



