Facts and Arguments for Darwin | Page 7

have thrown it aside as being completely foreign to the
developmental series which I was tracing, if the idea of early
Naupliiform stages of the higher Crustacea, which indeed I did not
believe to be still extant, had not at the moment vividly occupied my
attention.
And if I had not long been seeking among the Edriophthalma for traces
of the supposititious Zoea-state, and seized with avidity upon
everything that promised to made this refractory Order serviceable to
me, Van Beneden's short statement could hardly have affected me so
much in the manner of an electric shock, and impelled me to a renewed
study of the Tanaides, especially as I had once before plagued myself
with them in the Baltic, without getting any further than my
predecessors, and I have not much taste for going twice over the same
ground.
CHAPTER 4.
SEXUAL PECULIARITIES AND DIMORPHISM.

Our Tanais, which in nearly all the particulars of its structure is an
extremely remarkable animal, furnished me with a second fact worthy
of notice in connection with the theory of the origin of species by
natural selection.
When hand-like or cheliform structures occur in the Crustacea, these
are usually more strongly developed in the males than in the females,
often becoming enlarged in the former to quite a disproportionate size,
as we have already seen to be the case in Melita. A better known
example of such gigantic chelae is presented by the males of the
Calling Crabs (Gelasimus), which are said in running to carry these
claws "elevated, as if beckoning with them"--a statement which,
however, is not true of all the species, as a small and particularly
large-clawed one, which I have seen running about by thousands in the
cassava-fields at the mouth of the Cambriu, always holds them closely
pressed against its body.
A second peculiarity of the male Crustacea consists not unfrequently in
a more abundant development on the flagellum of the anterior antennae
of delicate filaments which Spence Bate calls "auditory cilia," and
which I have considered to be olfactory organs, as did Leydig before
me, although I was not aware of it. Thus they form long dense tufts in
the males of many Diastylidae, as Van Beneden also states with regard
to Bodotria, whilst the females only possess them more sparingly. In
the Copepoda, Claus called attention to the difference of the sexes in
this respect. It seems to me, as I may remark in passing, that this
stronger development in the males is greatly in favour of the opinion
maintained by Leydig and myself, as in other cases male animals are
not unfrequently guided by the scent in their pursuit of the ardent
females.
Now, in our Tanais, the young males up to the last change of skin
preceding sexual maturity resemble the females, but then they undergo
an important metamorphosis. Amongst other things they lose the
moveable appendages of the mouth even to those which serve for the
maintenance of the respiratory current; their intestine is always found
empty, and they appear only to live for love. But what is most

remarkable is, that they now appear under two different forms. Some
(Figure 3) acquire powerful, long-fingered, and very mobile chelae, and,
instead of the single olfactory filament of the female, have from 12 to
17 of these organs, which stand two or three together on each joint of
the flagellum. The others (Figure 5) retain the short thick form of the
chelae of the females; but, on the other hand, their antennae (Figure 6)
are equipped with a far greater number of olfactory filaments, which
stand in groups of from five to seven together.
(FIGURE 3. Head of the ordinary form of the male of Tanais dubius (?)
Kr. magnified 90 times. The terminal setae of the second pair of
antennae project between the cheliferous feet.
FIGURE 4. Buccal region of the same from below; lambda, labrum.
FIGURE 5. Head of the rarer form of the male, magnified 25 times.
FIGURE 6. Flagellum of the same, with olfactory filaments, magnified
90 times.)
In the first place, and before inquiring into its significance, I will say a
word upon this fact itself. It was natural to consider whether two
different species with very similar females and very different males
might not perhaps live together, or whether the males, instead of
occurring in two sharply defined forms, might not be only variable
within very wide limits. I can admit neither of these suppositions. Our
Tanais lives among densely interwoven Confervae, which form a coat
of about an inch in thickness upon stones in the neighbourhood of the
shore. If a handful of this green felt is put into a large glass with clear
sea-water, the walls of the glass are soon seen covered with hundreds,
nay with thousands,