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 Euthycarcinus, 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 Euthycarcinus 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 Euthycarcinus with that 

 ancient type. The short head and large marginal eyes recall Sidncyia, and the grouping 

 of the appendages about the mouth also suggests that genus and Emcraldclla. 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 

 Limulava and those of Euthycarcinus, but the composite claws of Sidncyia 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 antennas are also locomotor 

 organs, and in most orders are biramous. The mandibles are biramous and usually with, 

 but sometimes without, a gnathobase. The maxillulae 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- 



