486 GEORGE H. CARPENTER. 
order and is present in every Nauplius larva. Surely, there- 
fore, the presumption is that the foliaceous limb has been 
elaborated from the biramous and not that the latter has 
been simplified from the former. Now, the Crustacean orders 
in which the biramous (naupliar) condition of the limbs is 
most strikingly preserved are the Copepoda, the Trilobita and 
the Leptostraca. The trunk-limbs of the last are often men- 
tioned as phyllopodous in character, but the foliaceous struc- 
tures on the thoracic leg of Nebalia are the exopodite and epi- 
podite of a typical crustacean appendage, while in Paranebalia 
—see Sars (25)—the exopodite is slender and fringed, and 
the epipodite quite small. Similarly, the close comparison 
often made between the Branchiopoda and the Trilobita is 
misleading, for trilobitan lmbs—according to Beecher (1)— 
are biramous, and their foliaceous condition in the tail-region 
is due to a flattening of the segments of a typical endopodite 
(20, p. 217, fig. 35), not to the presence of numerous endites 
like those of Apus. My ideal ancestral crustacean is not 
therefore a Branchiopodan, but a form combining the most 
primitive characters of Triarthrus, Nebalia and Calanus. In 
this view I am largely in agreement with Sars (25), who 
suggests that the Nebaliide sprang from Copepod-lke 
ancestors, and that the Branchiopoda are a “ considerably 
modified ’’ offshoot (less primitive than the Leptostraca) from 
the same stem. I cannot follow Sars, however, in denying near 
relationship between the Leptostraca and the Malacostraca. 
He would derive the latter group independently from 
naupliiform ancestors, but the generally accepted view that 
the Leptostraca are to some extent ancestral to the Mala- 
costraca has much evidence in its favour. I have already 
suggested (p. 475, supra) how the two hindmost abdominal 
segments have undergone fusion in the Malacostraca as in the 
Insecta. 
If we accept the nauplius larva as representing the ancestral 
stock of the Crustacea, and therefore of all Arthropods, and 
if we follow many special students of the Crustacea in con- 
sidering the Copepoda as the most primitive living order of 
