APPENDAGES AND NERVOUS SYSTEM OF APUS CANCRIFORMIS. 3409 
Oxsservations and Reriections on the ApPENDAGES and on 
the Nervous System of Arus caNncrirorMis. By E. 
Ray Lanxester, M.A., F.R.S., Jodrell Professor of 
Zoology in University College, London. (With 
Plate XX.) 
APUS CANCRIFORMIs is in many respects one of the most 
important of the Crustacea. Not only is it of exceptionally 
large size for one of the Entomostraca, and therefore suited 
to anatomical investigation, but it possesses peculiarities of 
organisation which mark it out (together with its immediate 
congeners, the Phyllopoda) as an archaic form, probably 
standing nearer to the extinct ancestors of the Crustacea 
than any other living members of the group. 
The almost world-wide distribution of the genus Apus 
(Greenland, Tasmania, New Zealand, Australia, Europe, 
North America, West Indies, Affghanistan, China), and its 
fresh-water habit, tend to confirm the conclusion as to its 
high antiquity. 
But it is not only for such reasons that Apus has claims 
on our special attention. Its great abundance in certain 
localities renders it especially suitable for study as a type 
or standard of the organisation of the lower Crustacea, and 
it is on this account especially to be desired that an accurate 
account of its structure should be accessible. Strange as it 
appears, it is yet the fact that such an account does not 
exist, and that recent writers of authority have given diver- 
gent and erroneous accounts of such prominent features in 
the structure of Apus as the antenne, jaws, and thoracic 
appendages. 
In 1841 E. G. Zaddach published a memoir entitled ‘ De 
Apodis cancriformis anatome et historia evolutionis,’ which 
is a most carefully worked out and admirably illustrated 
monograph, worthy to be placed alongside the similar studies 
of Arthropod anatomy, which were produced at about the 
same period by George Newport. 
Zaddach’s most valuable observations refer to the internal 
anatomy and to the development of Apus from the egg, but 
the problems relative to the morphology of the appendages 
and the reciprocal relationships of the great groups of Ar- 
thropoda which now occupy attention had not in 1841 come 
into prominence, and accordingly we do not find his observa- 
tions upon the various appendages altogether equal in value 
to the rest of his work. 
Since the date of Zaddach’s monograph the only writers 
