190 The America ti (reohxjist. September, 1*95 
tion of the ceplialic i)()rtioii of the naiiplius.or protouaiipliiis is 
the dorsal shield which is present in many forms, and is consid- 
ered [vide Bernard^i) as a dorsal fold of the tiftli segment. So 
that in reviewing the naiiplius structures, we find here and 
there evidences of the entire series of head segments. 
Now, since the protaspis fulfils the rquirements hy having 
five well-developed cephalic segments, and is besides the oldest 
crustacean larva known, it is believed that, in so far, at least, it 
represents the primitive ancestral larval form for the class. 
The nauplius, therefore, is to be considered as a derived larva 
modified by adaptation. 
Other variations in the characters of the nauplius occur, but 
as they have clearlj' originated (a) from the parasitic habits of 
the adult, (6) from embryonic conditions, or (c) from earlier 
inheritance, the}" need not enter into consideration here. Such, 
for example, are [(i) the absence of an intestine in SaccHliud, 
(b) the absence of the median eye in Daphiiia and MoiiiH, and 
(e) the bivalve shell in Cypris. The larval stages of other, and 
especially later and higher groups of arthropods, oii'er more 
considerable differences and need not enter into this discus- 
sion, which is aimed chiefly to establish the genetic relationship 
between the protaspis of trilobites and the nauj^lius of re- 
cent Crustacea. 
VIII. Summary. 
Barrande first demonstrated the metamorphoses of trilobites 
in 1849, and recognized four orders of development, which 
ai'e now shown to be stages of growth of a single larval form. 
A common early larval form is recognized and called the 
protaspis. 
The protaspis has a dorsal shield, a cephalic portion com- 
posed of five fused segments and a pygidial portion consist- 
ing of the anal segment with one or more fused segments. 
The simplest protaspis stage is found in the Cambrian gen- 
era of trilobites. During later geological time it acquired 
additional characters by earlier inlieritance and became mod- 
ified, though retaining its [)entamerous glabella and small ab- 
dominal portion. 
Some of these acquired characters of the dorsal shield are 
the free-cheeks, the eyes, the eye-line, the- genal angles and 
the ornaments of the test. The free-cheeks and eyes moved 
to the dorsum fi-om tlie ventriim. 
