_Historical Development of the Crab-fish_ (Crustacea).—The two plates illustrate the development of the different Crustacea from the nauplius, their common primæval form. On Plate XI. six Crustacea, from six different orders, are represented in a fully developed state, whereas on Plate X. the early nauplius stages are given. From the essential agreement between the latter we may, on the ground of the fundamental law of biogeny, with full assurance maintain the derivation of the different Crustacea from a single, common primary form, a long since extinct Nauplius, as was first shown by Fritz Müller in his excellent work “Für Darwin.”(16) Plate X. represents the _early nauplius stages_ from the ventral side, so that the three pairs of legs, on the short, three-jointed trunk are distinctly visible. The first of these pairs of legs is simple and unsegmented, whereas the second and third pairs are forked. All three pairs are furnished with stiff bristles, which, through the paddling motion of the legs, serve as an apparatus for swimming. In the centre of the body, the perfectly simple, straight intestinal canal is visible, possessing a mouth in front, and an anal orifice behind. In front, above the mouth, lies a simple, single eye. All the six forms of nauplius entirely agree in all these essential characteristics of organization, whereas the six fully developed forms of Crustacea belonging to them, Plate XI., are extremely different in organisation. The differences of the six nauplius forms are confined to quite subordinate and unessential relations in regard to size of body, and the formation of the covering of the skin. If they could be met with in this form in a sexually mature condition, no zoologist would hesitate to regard them as six different species of one genus. (Compare vol. ii. p. 175.) Plate XI. represents those fully developed and sexually mature forms of Crustacea, as seen from the right side, which have ontogenetically (hence also phylogenetically) developed out of the six kinds of nauplius. Fig. _A c_ shows a freely swimming fresh-water crab (Limnetis brachyurus) from the order of the Leaf-foot Crabs (Phyllopoda), slightly enlarged. Of all the still living Crustacea, this order, which belongs to the legion of Gill-foot Crabs (Branchiopoda), stands nearest to the original, common primary form of nauplius. The Limnetis is enclosed in a bivalved shell, like a mussel. Our drawing (which is copied from Grube) represents the body of a female animal lying in the left shell; the right half of the shell has been removed. In front, behind the eye, we see the two feelers (antennæ), and behind them the twelve leaf-shaped feet of the right side of the body, behind on the back (under the shell), the eggs. Above, in front, the animal is fixed to the shell.