142 TPIE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
from the Hypoparia, continuing in the pelagic habitat. At first, most of the trilobites seem 
to have led a crawling existence, but about Middle Cambrian time they began to go back par- 
tially to the ancestral swimming habits, and retained some of the trunk segments to form 
a larger pygidium. The functional importance of the pygidium explains why it can not be 
used successfully in making major divisions in classification. Nearly related trilobites may 
be adapted to diverse methods of life. 
EVOLUTION WITHIN THE CRUSTACEA. 
The question naturally arises as to whether the higher Crustacea were derived from 
some one trilobite, or whether the different groups have been developed independently from 
different stocks. The opinion that all other crustaceans could have been derived from an 
Apus-like form has been rather generally held in recent years, but Carpenter (1903, p. 334) 
has shown that the leptostracan, Nebalia, is really a more primitive animal than Apus. He 
has pointed out that in Leptostraca the thorax bears eight pairs of simple limbs with 
lamelliform exopodites and segmented endopodites, while the abdomen of eight segments has 
six pairs of pleopods and a pair of furcal processes, so that only one segment is limbless. 
Contrasted with this are the crowded and complicated limbs of the anterior part of the 
trunk of Apus, and the appendage-less condition of the hinder portion. Further, a compari- 
son between the appendages of the head of Nebalia and those of Apus shows that the former 
are the more primitive. The antennules of Nebalia are elongate, those of Apus greatly re- 
duced; the mandible of Nebalia has a long endopodite, and Carpenter points out that from it 
either the malacostracan mandible with a reduced endopodite or the branchiopodan mandible 
with none could be derived, but that the former could not have arisen from the latter. 
The maxillae of Apus are also much the more specialized and reduced. 
Nebalia being in all else more primitive than Apus, it follows that the numerous ab- 
dominal segments of the latter may well have arisen by the multiplication of an originally 
moderate number, and the last trace of primitiveness disappears. 
It is now possible to add to the results obtained from comparative morphology the testi- 
mony of palaeontology, already outlined above, and since the two are in agreement, it must 
be admitted that the modern Branchiopoda are really highly specialized. 
As has already been pointed out, Hymenocaris, the leptostracan of the Middle Cambrian, 
has very much the same sort of appendages as the Branchiopoda of the same age, both 
being of the trilobite type. Which is the more primitive, and was one derived from the 
other ? 
The Branchiopoda were much more abundant and much more highly diversified in 
Cambrian times than were the Leptostraca, and, therefore, are probably older. Some of the 
Cambrian branchiopods were without a carapace, and some were sessile-eyed. These were 
more trilobite-like than Hymenocaris. Many of the Cambrian branchiopods had developed 
a bivalved carapace, though not so large a one as that of the primitive Leptostraca. The 
present indications are, therefore, that the Branchiopoda are really older than the Leptos- 
traca, and also that the latter were derived from them. It seems very generally agreed that 
the Malacostraca are descended from the Leptostraca, and the fossils of the Pennsylvanian 
supply a number of links in the chain of descent. Thus, Pygoccphalus cooperi, with its 
brood pouches, is believed by Caiman (1909, p. 181) to stand at the base of the Peracaridan 
series of orders, and Uronectes, Palaocaris, and the like are -Palaeozoic representatives of the 
Syncarida. Others of the Pennsylvanian species appear to tend in the direction of the Sto- 
