THE NAUPLIUS LARVA. 1 75 



different orders of Crabs, a Leaf-footed Crab (Limnetis, 

 Fig. A c) ; a Stalked Crab (Lepas, Fig. X) c) ; a Root Crab, 

 (Sacculina, Fig. E c); a, Boatman Crab (Cyclops, Fig. 5 c) ; a 

 Fish Louse (Lernseocera, Fig. C c) ; and, lastly, a highly 

 developed Shrimp (Penevis, 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, etc. When, however, we 

 look at the earliest stages, or " nauplius," of these six different 

 classes, after they have crept out of the egg— 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 moie 

 from one another than would six different " good species " 

 of one genus. Consequently, we may with assurance infei- a 

 common derivation of all those orders from a common 

 Primaeval 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 

 foi*m — 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, 



