Savory - Arachnida 1977

98 II. DE ARACHNIDIS
An obvious assumption is that the land was in a condition more
favourable to sustain life than it had been at the earlier period, and
that, correspondingly, the new arrivals were better fitted to exploit the
improvements. And how better fitted? If we are right in guessing that
they had come not from the depths of the ocean, but from the shallower
or even from the littoral zone, they would already have become accustomed
to exposure during low tides, when the use of atmospheric
oxygen was essential. This is most likely to be possible for a small
animal, seeking shelter under rocks, stones or sea weeds. If the relatively
large surface of a small animal can be kept moist, the moisture may
dissolve enough oxygen to avert catastrophe.
The danger to the pioneers came, therefore, not so much from the
difficulty of respiration as from the risk of desiccation, as formidable
a threat then as it is today. Arachnida and other Arthropoda can avoid
its worst effects because of a layer of wax in the cuticle. In marine
organisms such a layer may have controlled the passage of water by
osmosis in and out of the body. This would have been particularly
valuable to esturine species, exposed to recurrent changes in the salinity
ofthe surrounding water. This might be claimed as an example ofpreselection
for survival in a new environment, where the immigrants would
swell the ranks of the animals, still in the majority, that are known
collectively as the cryptozoa.
Nor should the possibility be neglected that some of the survivors did
not leave the sea voluntarily, if one may use the term, but were carried
ashore by tidal waves or in spray during an onshore gale. Cataclasms of
this kind have been adduced by other zoologists to explain some of the
peculiarities of evolution and distribution. Small organisms, thus illtreated,
might survive the experience, while many others must have
failed to do so. It appears that 16 kinds succeeded.
These products of speculation and imagination may well be followed
by, and indeed contrasted with, more orthodox attempts to trace the
evolutionary steps by which the orders have become as distinguishable
as they are today. The Arachnida which combine the greatest number
of primitive features are the Palpigradi; it is also undoubted that the
greatest number of specialized features is nine or ten, the total found in
the Ricinulei. The many structural differences between these two extremes
guide us in our search, and the studies of Petrunkevitch ( 1949)
suggested one way of attempting this reconstruction of the past.
There is no arachnid in which all the prosomatic tergites are separate,
and in nearly all orders the prosoma is covered by a uniform carapace.
However, in the three orders Schizomida, Solifugae and Palpigradi there
is a large propeltidium in front, with smaller plates, the mesopeltidium
11. PHYLOGENY: EVOLUTION 99
and metapeltidium, behind it. This suggests that fusion of the tergites
has proceeded backwards from the head.
On the ventral surface the sternites ha,·e partly fused in some orders
to form a large sternum, or ha,·e disappeared. These changes are associated
with the monment of the mouth and the approach of the coxae
towards the centre. There is always a labium or lower lip, the sternite
of the second somite, and in the Schizomida, Uropygi and Palpigradi
there are persistent sternites behind it. The backward movement of the
mouth to a position between the pedipalpi, which can be clearly witnessed
during indi,·idual dewlopment, is common to all Arachnida.
In the extinct orders Haptopoda and Anthracomarti, as in the living
king crabs, it is between the coxae of the first pair oflegs. The coxae of
the pedipalpi and legs take shares, in varying degrees, in the mastication
of the prey, so that their approach to each other is not surprising, and
in Opiliones and Solifugae no remnant of sternites is usually visible.
The last, fundamentally the sixth, sternite sometimes persists between
the fourth coxae, a position which, in Solifugae, is occupied by the first
sternite of the opisthosoma.
In the opisthosoma changes have occurred in the first somite and
some of the posterior somites.
The first tergite may be lost, as in recent scorpions, or fused with the
carapace, as in the Opiliones. In the orders grouped as the Caulogastra
it is constricted or reduced in circumference, tc form the pedicel, seen
in its primitive form in the Palpigradi.
The last three somites show different changes. They may be rather
similarly constricted to fcrm the narrow region known as the pygidium,
to be seen in Uropygi, Schizomida and Ricinulei; but these regions are
not strictly homologous, for in the U ropygi the pygidium represents
somites 10, 11 and 12, in the Schizomida 9, 10 and 11, and in the Ricinulei
7, 8 and 9, or, perhaps, 8, 9 and 10. The last stage in the process
is seen in spiders, where the anal tubercle represents from one to three
persistent tergites, their sternites having vanished. In spiders other than
Liphistiidae only five opisthosomatic somites remain, and in mites there
is often no trace either of segmentation or of anal tubercle.
This reduction of the opisthosoma has been carried to its extreme
limit in the Pycnogonida, a characteristic which well defines the
relationship and the difference between the Pycnogonida and the
Arachnida.
These evolutionary changes in the construction of the arachnid body
may therefore be summarized by saying that in the prosoma there has
been a gradual fusion of the tergites proceeding from before backwards,
and in the opisthosoma a loss or fusion of somites proceeding from behind
forward. It may be added that these changes appear to have taken