The pedigree of the Arthropoda can on the whole be clearly made out from the palæontology, comparative anatomy, and ontogeny of its four classes, although here, as everywhere else, many details remain very obscure. Not until the history of the individual development of all the different groups has become more accurately known than it is at present, can this obscurity be removed. The history of the class of Gilled Insects, or Crabs (Carides), is at present that best known to us; they are also called encrusted animals (Crustacea), on account of the hard crust or covering of their body. The ontogeny of these animals is extremely interesting and, like that of Vertebrate animals, distinctly reveals the essential outlines of the history of their tribe, that is, their phylogeny. Fritz Müller, in his work, “Für Darwin,”(16) which has already been referred to, has explained this remarkable series of facts in a very able manner. [Illustration: _Nauplius. Youth-form of six Crab-fish._ _Pl. X._ A. Limnetis. B. Cyclops. C. Lernacocera. D. Lepas. E. Sacculina. F. Peneus.] [Illustration: _Adult form of the same six Crab-fish._ _Pl. XI._ A. Limnetis. B. Cyclops. C. Lernacocera. D. Lepas. E. Sacculina. F. Peneus.] The common primary form of all Crabs, which in most cases is even now the first to develop out of the egg, is originally one and the same, the so-called _Nauplius_. This remarkable primæval crab represents a very simple form of articulated animal, the body of which in general has the form of a roundish, oval, or pear-shaped disc, and has on its ventral side only three pairs of legs. The first of these is uncloven, the two subsequent pairs are forked. In front, above the mouth, lies a simple, single eye. Although the different orders of the Crustacean class differ very widely from one another in the structure of their body and its appendages, yet the early Nauplius form always remains essentially the same. In order to be convinced of this, let the reader look attentively at Plates X. and XI., a more detailed explanation of which is given in the Appendix. On Plate XI. we see the fully developed representatives of six different orders of Crabs, a Leaf-footed Crab (Limnetis, Fig. _A c_); a Stalked Crab (Lepas, Fig. _D c_); a Root Crab (Sacculina, Fig. _E c_); a Boatman Crab (Cyclops, Fig. _B c_); a Fish Louse (Lernæocera, 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, 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 different forms of Nauplius of these six orders differ no more from one another than would six different “good species” of one genus.