1036 Ls the Group Arthropoda a valid one ? [October, 
evident in the adult of all, forms, but they are almost in iriably 
found in the young at some stage of development.! In insects the 
biramous structure of limb is never found,’ the appendages in 
all Tracheata having a simple form consisting of a number of 
joints serially arranged. In the Crustacea the organs of respira- 
tion, when present, are either limbs modified for aerating the 
blood or are appendages borne on the limbs. In the insects, 
when the respiratory organs are present, we find air-tubes or 
trachee permeating all parts of the body. In some forms i 
. specialized organs for breathing are found, while in the higher 
Arachnida pulmonary sacs are found in addition to the tracheal _ 
system. To repeat, in the Crustacea the blood is brought tothe — 
oxygen, in the insect the air is carried to the blood. 
When we turn to the alimentary tract we find an equa 
marked contrast between the two. In the Crustacea the rimi- 
tive stomach (archenteron) is usually formed by an im n 
while in the insects this is never, so far as our present knowledge 
extends, the case. The various portions of the alimentary tra 
of the two groups are equally difficult to homologize; in fact, ; 
any attempts in this direction result in showing analogies rather 
differences in the insects and in the Crustacea as they Go in a% 
two portions of the animal kingdom. In the Crustacea we ai 
more or less developed liver, while among the insects suet ©” 
organ is not well differentiated, and on the other hand the salvat) 
glands and malpighian vessels of the Tracheata are a 5 
allel in the Crustacea. 
In their development the Crustacea and Insecta sho 
markedly their diverse characters. The segmentation ofe 
a rule, centrolecithal, but the importance of this similarit 
estimated when we reflect that in Gammarus locusta We 
total segmentation, while in the closely allied G. pulex 
tial. From this point on every stage of developm 
1 As will be seen farther on the writer does not consider Limulus @ 
? With the solitary exceptions of Pauropus and Eurypauzopth i 
biflagellate antenna is found, but which can hardly be regarded as 1 
rule. 
’The “ lung” of Birgus and some of the land crabs is a secondary > 
feature, and has no importance in this connection. 
e 
