We can thus, as I infer from Professor Owen's interesting description of
these parts, understand the strange fact that every particle of food and
drink which we swallow has to pass over the orifice of the trachea, with
some risk of falling into the lungs, notwithstanding the beautiful
contrivance by which the glottis is closed. In the higher Vertebrata the
branchiae have wholly disappeared--but in the embryo the slits on the sides
of the neck and the loop-like course of the arteries still mark their
former position. But it is conceivable that the now utterly lost branchiae
might have been gradually worked in by natural selection for some distinct
purpose: for instance, Landois has shown that the wings of insects are
developed from the trachea; it is therefore highly probable that in this
great class organs which once served for respiration have been actually
converted into organs for 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 another 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 of the sack, together with the small frena, serving for respiration.
Pages:
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296