1908. ] PACKARD-——CLASSIFICATION OF ARTHROPODA. 151 
We quite agree with Pocock’s opinion that Paleeophonus was not 
adapted for land and aérial respiration, but ‘‘ lived in the sea, prob- 
ably in shallow water, its strong, sharply-pointed legs being fitted 
for maintaining a secure hold on the bottom.”’ 
In conclusion, then, we would suggest, from our present knowl- 
edge of the Palzeopoda, that the group is a natural one, that the 
line of descent of the phylum from some Annelid-like worm was 
independent of that of the crustacean phylum, and that the affini- 
ties shown by morphology and embryology to exist between the 
Trilobita, Merostomata and Arachnida are so close that they form 
a tolerably definite series of interrelated classes. 
Phylum If. PancarRipA. Represented bya single class, Crus- 
tacea. ‘The phylum name is proposed for the reason that the group 
is so well circumscribed, none but the genuine Crustacea or Carides 
belonging to it, forms as to whose position in nature all zoologists 
are well agreed. 
In this group there is a decided advance over the Palzopoda in 
the differentiation of the appendages into from three to six kinds, 
with corresponding functions. In the lower or more primitive, 
though somewhat modified, group of Cladocera, such as Daphnia, 
there are two pairs of antennz, a pair of mandibles, of maxille, | 
and of legs or trunk-appendages, the whole performing four different 
functions; while in the Decapoda there are besides the antenne, 
mandibles and maxillz, three pairs of maxillipedes, five pairs of 
thoracic and six of abdominal legs, or appendages, in all performing 
six different kinds of functions—a degree of differentiation and 
specialization not exceeded by any other Arthropodan group. 
The members of the phylum show an increasing tendency, as we 
rise towards the more specialized forms, to a heteronomous segmen- 
tation and also to a wonderful transfer of parts headwards (cephal- 
ization), the cephalothorax being covered by a carapace formed by 
the hypertrophy or excessive development of the tergites of the 
second antennal and mandibular segments. In the Phyllocarida 
the cephalothorax is covered by a bivalvular carapace, with a weak 
adductor muscle; while in Apus it, in adaptation to its burrowing 
in soft mud, assumes the general shape of the shield of Limulus; 
while in the Estheridz the entire body is protected by the two 
valves, which are connected by a definite hinge and ligament. On 
the other hand, the head-shield of the Palzeopoda, as well as the 
pygidium when occurring, is the result of the simple fusion of the 
