esterly: eucalanus. / 



Another method of preparation which has given excellent results 

 and, I believe, offers unique opportunities for study, is that of mount- 

 ing the central nervous system and sense organs entire. This is made 

 possible by the fact that Eucalanus reaches a length of from G to 8 mm. 

 The entire dorsal portion of the body may be removed by a stroke of 

 the scalpel, thus leaving the nervous system, including the eye and a 

 large part of the nerves, in place upon the ventral body wall. Such 

 prepai-ations may be cleared and mounted in the usual ways, and if 

 enough of the body has been removed, the preparation will be of such 

 slight thickness that high powers of the oil-immersion may be used 

 without injuring the specimen. If the staining, or coloration, due to 

 vom Rath's mixture is not too deep, one may follow single nerve fibres 

 for long distances, both within the cord and outside it; and in any 

 ease the ganglia and nerves in the thoracic segments stand out with 

 almost diagrammatic clearness. In the region of the mouth the 

 muscles of the organs pass toward the median plane of the body and 

 overlie the cord so closely that it is impossible to remove them without 

 injury to the nervous structures. This disadvantage may be overcome 

 to a considerable extent by comparing many entire preparations and 

 supplementing them by series of frontal sections. 



Before passing to a description of the histological features of the 

 light recipient organs in Eucalanus, I wish to express my appreciation 

 of the guidance and criticism of Prof. E. L. Mark, under whose direc- 

 tion the work has been done. His continual attention to accuracy of 

 observation and deduction, and his conservative judgment, have 

 been invaluable. I also wish to acknowledge the opportunities en- 

 joyed at the laboratory of the Marine Biological I>aboratory of San 

 Diego, where all of the material was obtained, and where, except for 

 the staining and sectioning, it was all prepared. 



II. The Optic Apparatus. 



1. The Median, Uninverted, Eye. 



The eye of Eucalanus belongs to the type which has been designated 

 by many authors as an "x-shaped pigment spot". Excepting the in- 

 vestigations of Richard ('91), Claus ('91), and Hartog ('88), there ap- 

 pear to have been none based upon a study of microscopic sections of 

 the tripartite eyes in the Copepoda, unless we except the work of 

 Hesse (:01), which deals particularly with the type of nerve ending in 



