IIO THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
Cambrian Branchiopoda are more closely allied to the Trilobita than are the modern ones, 
but still the subclass is not so closely related to that group as has been thought. Modern 
A pus is certainly much less like a trilobite than has been supposed, and very far from being 
primitive. The Branchiopoda of the Middle Cambrian could have been derived from the 
trilobites by the loss of the pleural lobes, the development of the posterior margin of the 
cephalon to form a carapace, and the loss of the appendages from the abdominal segments. 
Modern branchiopods can be derived from those of the Middle Cambrian by the modifica- 
tion of the appendages through the reduction of the endopodite and exopodite and the 
growth of the endites and exites from the proximal segments. 
Carpenter (1903, p. 334), from his study of recent crustaceans, has already come to 
the conclusion that the Branchiopoda are not the most primitive subclass, and this opinion 
is strengthened by evidence derived from the Trilobita and from the Branchiopoda of the 
Middle Cambrian. 
COPEPODA. 
The non-parasitic Eucopepoda are in many ways much nearer to the trilobites than any 
other Crustacea. These little animals lack the carapace, and the body is short, with typi- 
cally ten free segments and a telson bearing caudal furcze. The head is composed of five 
segments (if the first thoracic segment is really the fused first and second), is often flat- 
tened, and lacks compound eyes. Pleural lobes are well developed, but instead of being 
flattened as in the trilobite, they are turned down at the sides or even incurved. A labrum 
is present. 
The antennules, antennre, and mandibles are quite like those of trilobites. The anten- 
nules are very long and made up of numerous segments. The antenna; are biramous, the 
junction between the coxopodite and basipodite is well marked, and the endopodite consists 
of only two segments. 
The mandibles are said to "retain more completely than in any other Crustacea the 
form of biramous swimming limbs which they possess in the nauplius." The coxopodites 
form jaws, while both the reduced endopodite and exopodite are furnished with long setae. 
The maxillulse are also biramous, but very different in form from those of the trilobite, 
and the maxilla? are phyllopodan. 
The first thoracic limb is uniramous and similar to the maxillae, but the five following 
pairs are biramous swimming legs with coxopodite, basipodite, exopodite, and endopodite. 
Both the exopodite and endopodite are shorter than in the trilobites, but bear setfe and spines. 
The last pair of thoracic limbs are usually modified in the male into copulatory organs. 
In some females they are enlarged to form plates for the protection of the eggs, in others 
they are unmodified. In still others they are much reduced or disappear. The abdomen 
is without appendages. 
The development in Copepoda is direct, by the addition posteriorly to the larval form 
(nauplius) of segments, and the appendages remain nearly unmodified in the adult. 
Altogether, the primitive Copepoda seem much more closely allied to the Trilobita than 
any other modern Crustacea, but unfortunately no fossil representative of the subclass 
has been found. This is not so surprising when one considers the habits and the habi- 
tat of most of the existing species. Many are parasitic, many pelagic in both fresh 
and marine waters, and many of those living on the bottom belong to the deep sea or fresh 
water. Most free-living forms are minute, and all have thin tests. 
