362 Dr. W. T. Caiman — The Appendages of Trilobites. 



is true that many writers even at the present day adopt the 

 classification of Crustacea into the two sub-classes Entomostraca 

 and Malacostraca, but this is due merely to the inertia of tradition. 

 The " Entomostraca " have no more claim to constitute a taxonomic 

 unity than have the " Invertebrata ". The groups included under 

 the name, the Branchiopoda, Ostracoda, Copepoda, and Cirripedia, 

 are no more closely related to one another than any one of them 

 is to the Malacostraca, and they should be treated as equivalent 

 sub-classes of the Crustacea. "When they are arranged in this 

 fashion it becomes clear that the Branchiopoda are the only sub-class 

 that can be regarded as having any direct relationship with the 

 Trilobites. Each of the other sub-classes has attained to a more 

 or less strictly defined number of trunk-somites and appendages. 

 Only the Branchiopoda, like the Trilobita, are, to use Lankester's 

 term, anomomeristic. The Malacostraca, in addition, have the trunk 

 appendages grouped in the two sharply defined " tagmata " belonging 

 to the thorax and abdomen. The Ostracoda, Copepoda, and Cirripedia 

 are, in different ways, highly specialized groups, and the only 

 characters which can be usefully considered in comparing them with 

 Trilobites are those that may be supposed to be inherited from the 

 common stock of Crustacea. Thus, it is legitimate to supplement 

 a comparison of the Trilobites with the Notostraca (Jtpus) by 

 a reference to the more generalized mouth-parts (biramous mandible- 

 palp, etc.) of certain Copepods. 



One of the pieces of evidence that has influenced opinion most 

 strongly in favour of the Crustacean affinities of the Trilobites is 

 afforded by Beecher's determination of the number of cephalic 

 appendages in Triarthrus. Behind the preoral antennules, he 

 found four pairs of appendages, each biramous and provided with 

 a gnathobase. Since the antennae of Crustacea are still postoral and 

 carry a masticatory process or gnathobase in the nauplius larva, 

 the correspondence of the postoral cephalic appendages of Triarthrus 

 with antenna?, mandibles, maxillulse, and maxillae seems to be 

 complete. In view of the great importance of this correspondence 

 it is much to be regretted that it has not been possible to confirm 

 it in any of the other species of Trilobites. Dr. Walcott, indeed, 

 repeatedly refers to four pairs of cephalic legs, but it is not clear 

 that the precise number could be determined in any case without 

 reference to the analogy of Triarthrus. 



The biramous form of the limbs in Triarthrus has been regarded, 

 with justice, as one of the main supports of Crustacean affinity. 

 The objection that the protopodite, to which the two rami are 

 attached, appears to be unsegmented, while in the Crustacea it is 

 usually composed of two, sometimes of three segments, need not be 

 regarded as insurmountable. Dr. Walcott definitely describes the 

 protopodite of the Trilobites as " consisting of a fused coxopodite 

 and basopodite ", but this would appear to be rather a probable 

 inference than an observed fact. 



The discovery of a pair of multiarticulate caudal filaments in 

 Neolenus is a new and weighty piece of evidence in favour of affinity 

 with Crustacea. In one form or another a caudal furca is found in 



