152 PACKARD—CLASSIFICATION OF ARTHROPODA. [April 3, 
segments. While the Phyllopoda are generally regarded as the most 
primitive group—their swimming limbs closely resembling those 
of Annelid worms—it may be questioned whether the Phyllocarida 
are not astill more primitive group; certainly they are the most 
composite or synthetic, and were the earliest known group of 
Crustacea. 
Crustacea are, like the Paleopoda, prosogoneate ; but when we 
take into account the fact that there is in the adult but a single pair 
of nephridia (the green gland), or in other forms (Phyllopoda) the 
shell gland, there has been a great reduction in the number of pairs 
from what may have been the ancestral type, while Limulus still 
retains four pairs. In all Crustacea the eggs are carried attached 
to the body of the parent, and never, as in trilobites and meros- 
tomes, deposited loosely in the sand. 
In their metamorphosis, which is a complete one in all the typi- 
cal forms, the larval stage of the lower Crustacea being a nauplius, 
we have another feature wanting in the Palzopoda. As is well 
known, the early embryo of Moina passes through a prenauplian 
stage like that of Annelida, and the indications are that the nau- 
plius is itself a derivation from the trochosphera stage of Annelid 
worms. te 
Now, as is well known, the most primitive groups or members of 
a group do not undergo transformations ; and in this respect the 
Pancarida are a later, more specialized group than the Paleeopoda. 
It will be remembered that the most primitive insects (Synaptera) 
do not undergo a metamorphosis, and if several of the lower orders 
of winged insects it is incomplete, there being no larval and pupal 
‘stages; in the Arachnida only the extremely modified Acarina 
undergo a slight metamorphosis. That of the Meropoda is slight. 
Enough has been stated, we think, to show that the Paleopoda 
are quite remote from the Pancarida, and that a union of the trilo- 
bites in the same class with the Crustacea brings about an unnat- 
ural association, and tends to an unnecessary amount of confusion. 
Dr. Kingsley regards the Trilobita as the more primitive sub- 
class of Crustacea, but we are unable to see any features in Crustacea 
which could have been derived from trilobites; there are no 
transitional forms, and the larval forms are widely distinct, as he 
has well shown. ‘The gap between the two groups is, on morpho- 
logical and embryological grounds, a very wide one. Already in 
the early Cambrian seas trilobites were a predominant type, while 
