PRIMITIVE CHARACTERISTICS OF TRILOBITES. 



Because of the many instances in which such extra growths arise, and because of the form 

 of the appendages of the Branchiopoda, it has been suggested that the primitive crustacean 

 leg must have been more complex than that of the trilobite. In looking over the Malacos- 

 traca, however, one is struck by the fact that epipodites generally arise where the exopo- 

 dites have become aborted or are poorly developed, and seem largely to replace them. The 

 coxopodite and basipodite are usually fused to form a protopodite, and a third segment 

 is sometimes present in the proximal part of the appendage. 



In the Branchiopoda are found the most complex crustacean limbs, and the ones most 

 difficult to homologize with those of trilobites. In recent years, Lankester's homologies 

 of the parts of the limbs of Apus with those of the Malacostraca have been quite gener- 

 ally accepted, and the appendages of the former considered primitive. Now that it is 

 known that the Branchiopoda of the Middle Cambrian (Burgessia et at.) had simple trilo- 

 bite-like appendages, it becomes necessary to exactly reverse the opinion in this matter. 

 The same homologies stand, but the thoracic limbs of Apus must be looked upon as highly 

 specialized instead of primitive. 



Fig. 34. One of the appendages of the 

 anterior part of the trunk of Apus, showing 

 the endites (beneath) and exites (above). 

 The proximal endite forms a gnathobase 

 which is not homologous with the gnatho- 

 base (or endobase) of the trilobite. Copied 

 from Lankester. Much enlarged. 



Lankester (Jour. Micros. Sci., vol. 21, 1881) pointed out that the axial part of the 

 thoracic limb of Apus (fig. 34) is homologous with the protopodite in the higher Crus- 

 tacea, that the two terminal endites corresponded to the exopodite and endopodite, and that 

 the other endites and exites were outgrowths from the protopodite analogous to the epip- 

 odites of Malacostraca. There seems to be no objection to retaining this interpretation, 

 but with the meaning that both endopodite and exopodite are much reduced, and their func- 

 tions transferred to numerous outgrowths of the protopodite. One of the endites grows 

 inward to form an endobase, the whole limb showing an attempt to return to the ancestral 

 condition of the trilobite. The limbs of some other branchiopods are not so easy to under- 

 stand, but students of the Crustacea seem to have worked out a fairly satisfactory compari- 

 son between them and Apus. 



The discovery that the ancestral Branchiopoda had simple biramous appendages instead 

 of the rather complex phyllopodan type is another case in which the theory of "recapitu- 

 lation" has proved to hold. It had already been observed that in ontogeny the biramous 

 limb preceded the phyllopodan, but so strong has been the belief in the primitive character 

 of the Apodida? that the obvious suggestion has been ignored. Even in such highly special- 

 ized Malacostraca as the hermit crabs the development of certain of the limbs illustrates the 

 change from the schizopodal to the phyllopodan type, and Thompson (Proc. Boston Soc. 

 Nat. Hist., vol. 31, 1903, pi. 5, fig. 12) has published an especially good series of drawings 



