394 SEUOLIS, LIMULUS, BRANCHIPUS. 
A third example of this disposition, in an 
animal belonging to the same class of Crus- 
taceans, whereby the legs are reduced to soft 
paddles, and combine the functions of respi- 
ration with those of locomotion, is afforded by 
the Branchipus stagnalis, (Cancer stagnalis, 
Lin.), of our English ponds, (see PI. 45, Figs. 
3, e. 4, e. 5, e.) 
In the comparison here iliade 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 
horny transverse plates (Fig. 2, e. 2, e'. and 2, e".) supporting 
the fibres of the branchiae, and at the same time acting as paddles 
for swimming. The same disposition of laminated branchiae is 
found also in the Serolis, Fig. 7. e. Fig. 8, is a magnified 
representation of these laminated branchiae, very similar to those 
at Figs. 3, e. and 5, e. 
Thus while the Serolis (Fig. 7) presents an union of antennm 
and crustaceous legs with soft paddles bearing the Branchiae, we 
have in the Limulus (Fig. 2), a similar disposition of legs and 
paddles, and only slight traces of antennae ; in the Branchipus, 
(Figs. 3 and 5,) we find antennae, but no crustaceous legs; while 
the Trilobite, being without antennae, and having all its legs 
represented by soft paddles, as in Branchipus, is by the latter 
condition placed near Branchipus among the Entomostracous 
Crustaceans, in the Order of Branchiopods, 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 branchiae of Branchipus, performing the double office 
of feet and lungs. 
