128 The American Naturalist. [February; 
piration. Yet these are, in the opinion of many, not homol- 
ogous. In the Hexapoda they arise, ontogenetically, as in- 
pushings of the ectoderm of sides of the body, outside and above 
the line of the insertion of the limbs. Their method of growth, 
the general structure, etc., all point to their origin, as was 
pointed out by Chun (’75) from dermal glands which later 
assumed respiratory functions. The trachee of the Arachnids, 
on the other hand, have had a different origin. In those 
forms in which they have been studied, they arise as inpush- 
ings behind the temporary appendages on the abdomen. 
There is not a little evidence to show that they have arisen 
from gills borne on the posterior surfaces of these appendages, 
as in the Limulus of to-day ; that they have been pushed into 
the body, taking the form of lung books, a condition perma- 
nent in all the respiratory organs of the Scorpions and in those 
. of one or two somites of the Araneina; and then, coincidently 
with a reduction in the circulatory organs, they have penetra- 
ted farther and farther into the body. For the details of this 
process, as well as for the wonderful histological similarity 
between the embryonic gills of Limulus and lungs of Arachnids 
the reader is referred to my full paper. The “spiral threads” 
in the two cases are to be explained as mechanical in origin— 
corrugations give greater strength without excessively thick- 
ening the intima. Still, a third type of “trachea” 1s to be 
be found in the gills of the Oniscid Crustacea. These organs 
have become adapted for aerial respiration, and, in connection 
with this change, the organs have been permeated by branches 
of minute tubes, lined with a chitinous intima, produced by 
inpushings of the outer body wall. These trachee cannot be 
regarded by the strongest advocate of the naturalness of the 
“ Tracheates ” as homologous (i. e., homogenous) with those of 
the Hexapods. I have made a number of, as yet unpublished, 
observations on these organs in Porcillio. Leydig described 
them in detail some years ago (’78). The peculiar structures 
in the genus Tylos as described by Henri Milne-Edwards (4 : 
p- 187-8) should be considered in this connection. 
It is only recently that the existence of nephridia in the Ar- 
thropoda has been placed beyond a doubt. The earlier students 
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