| → of the sack, including 1866 |
| sack, including 1859 1860 1861 |
| of the sack, together with 1869 1872 |
|
| → in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. 1866 |
| large folded branchiæ. 1859 1860 |
| in the same relative position, large, much folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. 1861 |
| in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and by all other naturalists who have treated on the subject. 1869 |
| in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered by all naturalists to act as branchiæ. 1872 |
|
| → it need not be 1866 1869 1872 |
| I do not 1859 1860 1861 |
|
| → the two little 1861 1866 1869 1872 |
| little 1859 1860 |
|
|
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
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 branchiæ, the whole surface of the body and
→of the sack, including
the small frena, serving for respiration. The Balanidæ or sessile cirripedes, on the other hand, have no ovigerous frena, the eggs lying loose at the bottom of the sack,
the well-enclosed shell; but they
→in the same relative position with the frena, large, much-folded membranes, which freely communicate with the circulatory lacunæ of the sack and body, and which have been considered to be branchiæ by Prof. Owen and all other naturalists who have treated on the subject. Now I think no one will dispute that the ovigerous frena in the one family are strictly homologous with the branchiæ of the other family; indeed, they graduate into each other. Therefore
→it need not be
that
→the two little
folds of skin, which originally served as ovigerous frena, but which, likewise, very slightly
act of respiration, have been gradually converted by natural selection into branchiæ, simply through an increase in their size and the obliteration of their adhesive glands. If all pedunculated cirripedes had become extinct, and they have
suffered far more extinction than have sessile cirripedes, who would ever have imagined that the branchiæ in this latter family had originally existed as organs for preventing the ova from being washed out of the sack?
|
