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. 
‘“‘TIn 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 primev 
form of limulus have possessed such an intermediate charaeter, oF 
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- 
servation of parts so delicate as the visual organs of animals that 
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