112 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
The non-parasitic copepods have typically ten (eleven) free segments, including the 
telson, and the four abdominal segments are much more slender than the six in front of 
them. In this respect the agreement is striking, and the presence of five pairs of appen- 
dages in the head and six free segments in the thorax is a more primitive condition than 
in modern forms where the first two thoracic segments are apparently fused (Caiman, 1909, 
P- 73)- 
The large compound eyes of this animal are of course not present in the copepods, but 
as vestiges of eyes have been found in the young of Calanus, it is possible that the ancestral 
forms had eyes. 
The greatest difficulty is in finding a satisfactory explanation of the appendages. The 
general condition is somewhat more primitive than in the copepods, for all the appendages 
are biramous, while in the modern forms the maxillipeds are uniramous and the sixth pair 
of thoracic appendages are usually modified in the male as copulatory organs. In the cope- 
pods the modification is in the direction of reduction, both endopodites and exopodites usu- 
ally possessing fewer segments than the corresponding branches in the trilobites. The 
endopodite of Euthycarcimis, . on the contrary, possesses, if Handlirsch's interpretation is 
correct, twice as many segments as the endopodite of a trilobite. If the Copepoda are 
descended from the trilobites, as everything tends to indicate, then Euthycarcimis is certainly 
not a connecting link. The only truly copepodan characteristic of this genus is the agree- 
ment in number and disposition of free segments. The division into three regions instead 
of two, the compound eyes, and the structure of the appendages are all foreign to that group. 
With the Limulava fresh in mind, one is tempted to compare Euthycarcimis with that 
ancient type. The short head and large marginal eyes recall Sidneyia, and the grouping 
of the appendages about the mouth also suggests that genus and Emeraldella. In the Limu- 
lava likewise there is a contraction of the posterior segments, although it is behind the 
ninth instead of the sixth. There is no likeness in detail between the appendages of the 
L'imulava and those of Euthycarcinus, but the composite claws of Sidneyia show that in 
this group there was a tendency toward the formation of extra segments. 
If this fossil had been found in the Cambrian instead of the Triassic, it would prob- 
ably have been referred to the Limulava, and is not at all impossible that it is a descendant 
from that group. As a connecting link between the Trilobita and Copepoda it is, however, 
quite unsatisfactory. 
OSTRACODA. 
The bivalved shell of the Ostracoda gives to this group of animals an external appear- 
ance very different from that of the trilobites, but the few appendages, though highly modi- 
fied, are directly comparable. The development, although modified by the early appearance 
of the bivalved shell within which the nauplius lies, is direct. Imperfect compound eyes 
are present in one family. 
The antennules are short and much modified by functioning as swimming, creeping, or 
digging organs. They consist of eight or less segments. The antenna? are also locomotor 
organs, and in most orders are biramous. The mandibles are biramous and usually with, 
but sometimes without, a gnathobase. The maxillula; are likewise biramous but much 
modified. 
The homology of the third post-oral limb is in question, some considering it a maxilla 
and others a maxilliped. It has various forms in different genera. It is always much modi- 
