298 SEROLIS, LIMULIS, BRANCHIPUS. 



A third example of this disposition, in an animal belong- 

 ing to the same class of Crustaceans, whereby the legs are 

 reduced to soft paddles, and combine the functions of respi- 

 ration with those of locomotion, is afforded by the Branchi- 

 pus stagnahs, (Cancer stagnalis, Lin.,) of our EngUsh ponds, 

 (see PI. 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 cognizance, 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, w^hile 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 parts 

 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 func- 

 tions of paddles and lungs. The advocate for the theory 

 of the derivation of existing more perfect species, by suc- 

 cessive 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 Ento- 

 inostracous Crustaceans, in the order of Branchlopods, whose feet are repre- 

 sented by ciliated paddles, combining the functions of respiration and natation- 

 At PI. 45. Fig. 3. e, Fig. 4. e. Fig. 5. e, represent the soft branchite of 

 Branchipus, performing the double office of feet and lungs. 



