188 The American Geoloyist. September, 1895 
and tile mandibles ; the first pair is uniramous and sensoiy in 
I'linction ; the second and third pairs are biranioiis, swimming 
appeiuhiges; body usually unsegmented ; anteriorly there is a 
single median eye, and a large labnim, or upper lip; an ali- 
meiitray canal bent anteriorly, and ending in an anus near the 
posterior entl of the body : a dorsal shield ; the second pair of 
anteniue are innervated from a sub-ffisophageal ganglion. 
Frontal sense organs and a rudimentary metastoma are some- 
times ])resent. The trunk and abdominal regions are not gen- 
erally differentiated. 
Balfour^ remarks of the nauplius that : "In most instances 
it does not e.rr/r//// conform to the above type, and the diver- 
gences are more considerable in the Phyllopods than in most 
other groups." This variation is indeed quite marked among 
nearly all the groups besides the phyllopods and furnishes the 
facts for the conclusion, that the hexa])odous condition is not 
primitive. 
On plate X are rei)resented some of the leading types of 
nauplius structure, taken chiefly from the excellent compila- 
tion by Faxon.-' Bearing in mind the typical and average 
characters of this larva, some of the variations will be briefly 
reviewed. 
The nau])lius of J/ms, represented in plate X, figure 2, 
shows the rudiments of Ave trunk segments, which in a later 
stage ( ttgure 3) develoj) phyllopodiform appendages belonging 
to sixth, seventh, and eighth pairs of limbs. They are the an- 
terior trunk api)endages and appear at a time when the fourth 
cephalic pair is a mere rudiment while the fifth is entirely un- 
developed. The fourth and fifth pairs of head appendages evi- 
dently must have some existence, though undeveloped in the 
nauplius. The physical conditions of nauplius life probably do 
not require them, and they therefore remain for a time quies- 
cent or undeveloped. 
In figures 4, 5, 8, and 6, respectively, of Branrhiims. Arfeiiu'a, 
Lepfodora, and LimiKo'da, the first pair of appendages becomes 
progressively shortened, until, in the last, they almost disap- 
pear. LeplodDfd (figure 8) and LepidKrvs (figure 7) also have 
rudimentary trunk segments and appendages (//). Figui'es 9 and 
10, of Diiphiild and Moiita (from summer eggs), show how rudi- 
mentary the nauplius appendages may become when this stage 
