28 Remarks on the Trilobite, 



among the entomostracous crustaceans, in the order of branchio- 

 pods, whose feet are represented by ciliated paddles, combining 

 the functions of respiration and natation, 



" 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, lim- 

 ulus, 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, 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 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 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 forms 

 of more perfect crustaceans may, during the lapse of ages, have 

 been derived; but according to this hypothesis, we ought no 

 longer to find the same simple condition as that of the trilobite 

 still retained in the living branchipus, nor should the primeval 

 form of limulus have possessed such an intermediate character, or 

 have remained unadvanced in the scale of organization, from its 

 first appearance in the carboniferous series, through the midway 

 periods of the secondary formations, unto the present hour. 



" Besides the above analogies between the trilobites and certain 

 forms of living crustaceans, there remains a still more important 

 point of resemblance in the structure of their eyes. This point 

 deserves peculiar consideration, as it affords the most ancient, and 

 almost the only example yet found in the fossil world, of the pre- 



so 



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