1885.] The Inter-Relationships of Arthropods. 563 
tacea another pair of glands occur, the antennal or green glands 
which also appear to belong to the same series. 
A further point is also to be mentioned. In both Acerata and 
Crustacea the genital ducts open at the base of a pair of appen- 
dages near the middle of the body, and although that exact 
homology is lacking as to position which is seen in the case of the 
segmental organs, still there is enough similarity to make one 
think that here, as well as in other forms, a pair of segmental 
ducts has become modified to subserve the purposes of the geni- 
tal system. 
Did space allow, these comparisons could be prolonged almost 
indefinitely, showing that while there is a general resemblance 
between the Acerata and the Crustacea, there exists a much closer 
one between Limulus and the arachnids. If we turn now to a 
comparison of the Acerata, or even the arachnids proper, with 
the hexapods, we are at once struck with the important differ- 
ences between them ; differences which prove that the two groups 
have but little in common, and that, so far as these two are con- 
cerned, the division Tracheata is an artificial and not a natural 
‘aes 
We have already alluded to one important difference between 
the trachez in the two groups. A few other remarks may prove 
of value. Trachez are internal tubes for conducting air to the 
tissues of the body. They are not confined to the “ Tracheata” 
but occur in some of the terrestrial Crustacea. This was first 
pointed out by Lereboullet in 1851 in the sow-bugs (Oniscidz), 
and more lately it has been shown that these trachez which are 
developed inside the branchial lamellz are lined with a cuticle 
which is raised into folds, comparable to the so-called spiral fila- 
ment in the trachez of the hexapods. The inference to be drawn 
is that trachez in the arthropods are not of phylogenetic signifi- 
cance, but have arisen from a necessity of conveying air to the 
blood and tissues in an air-breathing form. The thickenings of 
the cuticular wall, whether spirally or irregularly arranged, are 
intended to prevent the collapse of an otherwise delicate tube. 
In both spiders and hexapods there are developed from the 
hinder division of the digestive tract excretory organs which are 
known as urinary or Malpighian tubules. The writer holds that 
these are not to be regarded as indicating any especial affinity 
between the two groups, but like the trachez are produced by 
