In the higher Vertebrata the branchiae have wholly
disappeared--the slits on the sides of the neck and the loop-like
course of the arteries still marking in the embryo their former
position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some
quite distinct purpose: in the same manner as, on the view entertained
by some naturalists that the branchiae and dorsal scales of Annelids
are homologous with the wings and wing-covers of insects, it is
probable that organs which at a very ancient period served for
respiration have been actually converted into organs of flight.
In considering transitions of organs, it is so important to bear in
mind the probability of conversion from one function to another, that
I will give one more instance. Pedunculated cirripedes have two minute
folds of skin, called by me the ovigerous frena, which serve, through
the means of a sticky secretion, to retain the eggs until they are
hatched within the sack. These cirripedes have no branchiae, the whole
surface of the body and sack, including the small frena, serving for
respiration. The Balanidae or sessile cirripedes, on the other hand,
have no ovigerous frena, the eggs lying loose at the bottom of the
sack, in the well-enclosed shell; but they have large folded
branchiae. Now I think no one will dispute that the ovigerous frena in
the one family are strictly homologous with the branchiae of the other
family; indeed, they graduate into each other.
Pages:
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239