THE NAUPLIUS LARVA. 175 
different orders of Crabs, a Leaf-footed Crab (Limnetis, 
Fig. A c); a Stalked Crab (Lepas, Fig. D c); a Root Crab, 
(Sacculina, Fig. # c); a Boatman Crab (Cyclops, Fig. Bc) ; a 
Fish Louse (Lernzeocera, Fig. C c); and, lastly, a highly 
_ developed Shrimp (Peneus, Fig. F c) These six crabs vary 
very much, as we see, in the entire form of body, in the 
number and formation of the legs, ete. When, however, we 
look at the earliest stages, or “ nauplius,” of these six different 
classes, after they have crept out of the ege—those marked 
with corresponding letters on Plate X. (Fig. A n—F n)—we 
shall be surprised to find how much they agree. The differ- 
ent forms of Nauplius of these six orders differ no more 
from one another than would six different “good species” 
of one genus. Consequently, we may with assurance infer a 
common derivation of all those orders from a common 
Primeval Crab, which was essentially like the Nauplius of 
the present day. 
The pedigree on p. 177 will show how we may at 
present approximately conceive the derivation of the 
twenty orders of Crustacea enumerated on p. 176, from the 
common primary form of the Nauplius. Out of the Nauplius 
form—which originally existed as an independent genus— 
the five legions of lower Crabs developed as diverging 
branches in different directions, which in the systematic 
survey of the class are united as Segmented Crabs (Entomos- 
traca). The higher division of Mailed Crabs (Malacostraca) 
have likewise originated out of the common Nauplius form. 
The Nebalia is still a direct form of transition from the 
Phyllopods to the Schizopods, that is, to the primary form 
of the stalk-eyed and sessile-eyed Mailed Crabs. The 
Nauplius at this stage gives rise to another larva form, 
