PRIMITIVE CHARACTERISTICS OF TRILOBITES. 127 
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 ct al.) 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. ScL, 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 Apodidte 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 
