Fig. _E c_ represents a parasitic sack-crab (Sacculina purpurea) from the order of Root-crabs (Rhizocephala). These parasites, by adaptation to a parasitical life, have developed out of Barnacle crabs (Fig. _D c_), much in the same way as the fish-lice (_C c_), out of the freely swimming Oar-legged crabs (_B c_). However, the suppression, and the subsequent degeneration, of all of the organs, has gone much further in the present case than in most of the fish-lice. Out of the articulated crab, possessing legs, intestine, and eye, and which in an early stage as nauplius (_E n_, Plate VIII.), swam about freely, there has developed a formless, unsegmented sack, a red sausage, which now only contains sexual organs (eggs and sperm) and an intestinal rudiment. The legs and the eye have completely disappeared. At the posterior end is the opening of the genitals. From the mouth grows a thick bunch of numerous tree-shaped and branching root-like fibres. These spread themselves out (like the roots of a plant in the ground) in the soft hinder part of the body of the hermit-crab (Pagurus), upon which the root-crab lives as a parasite, and from which it draws its nourishment. Our drawing (_E c_), a copy of Fritz Müller’s, is slightly enlarged, and shows the whole of the sausage-shaped sack-crab, with all its root-fibres, when drawn out of the body upon which it lives. Fig. _F c_ is a _shrimp_ (Peneus Mülleri), from the order of _ten-foot_ crabs (Decapoda), to which our river cray-fish, and its nearest relative, the lobster, and the short-tailed shore-crabs also belong. This order contains the largest and, gastronomically, the most important crabs, and belongs, together with the mouth-legged and split-legged crabs, to the legion of the stalk-eyed mailed crabs (Podophthalma). The shrimp, as well as the river crab, has in front, on each side below the eye, two long feelers (the first much shorter than the second), then three jaws, and three jaw-feet, then five very long legs (the three fore ones of which, in the Peneus, are furnished with nippers, and the third of which is the longest). Finally, on the first five joints of the hinder part of the body there are other five pairs of feet. This shrimp, which is one of the most highly developed and perfect crabs, originates (according to Fritz Müller’s important discovery) out of a nauplius (_F n_ Plate VIII.), and consequently proves that the higher Crustacea have developed out of the same form as the lower ones, namely, the nauplius. (Compare vol. ii. p. 175.) PLATES XII. AND XIII. (_Between pages 200 and 201, Vol. II._)