A third example of this disposition, in an animal belonging to the same class of Crustaceans, whereby the legs are reduced to soft paddles, and combine the functions of respiration with those of locomotion, is afforded by the Branchipus stagnalis, (Cancer stagnalis, Lin.,) of our English ponds, (see Pl. 45, Figs. 3, e. 4, e. 5, e.)
In the comparison here made between four different families of Crustaceans, for the purpose of illustrating the history of the long extinct Trilobites, by the analogies we find in the Serolis, Limulus, and Branchipus; we have a beautiful example, taken from the extreme points of time of which Geology takes cognisance, of that systematic and uniform arrangement of the Animal Kingdom, under which every family is nearly connected with adjacent and cognate families. Three of the families under consideration are among the present inhabitants of the water, while the fourth has been long extinct, and occurs only in a fossil state. When we see the most ancient Trilobites thus placed in immediate contact with our living Crustaceans, we cannot but recognise them as forming part and parcel of one great system of Creation, connected through its whole extent by perfect unity of design, and sustained in its minutest by uninterrupted harmonies of organization.
We have in the Trilobites an example of that peculiar,
and, as it is sometimes called, rudimentary development
of the organs of locomotion in the Class Crustaceans,
whereby the legs are made subservient to the double functions
of paddles and lungs, The advocate for the theory
of the derivation of existing more perfect species, by successive
changes from more simple ancient forms, might
imagine that he sees in the Trilobite the extinct parent
stock from which, by a series of developments, consecutive
pus, is by the latter condition placed near Branchipus among the Entomostracous Crustaceans, in the order of Branchiopods, whose feet are represented by ciliated paddles, combining the functions of respiration and natation. At Pl. 45. Fig. 3. e, Fig. 4. e, Fig. 5. e, represent the soft branehia: of Branehipus, performing the double office of feet and lungs.
