ESTERLY: EUCALANUS. 7 
Another method of preparation which has given excellent results 
and, | 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 6 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 
preparations 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 
case 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 Laboratory 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. Tae Mepran, 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 
