Savory - Arachnida 1977

100 11. DE ARACHNIDIS
place independently, and that they have tended towards a simplification
of the primitive 18-somite pattern. At the same time specializations
have appeared, making for greater distinctions between the different
orders. Many of these arc seen to be associated with environment and
habit. As might be expected they are fewer in the orders with persisting
segmentation, the scorpions, Uropygi and Amblypygi, and more
numerous in those with the contrasting type of body, the spiders, mites
and Opiliones.
Thus it seems possible to make reasonable suggestions as to the types
of change that evolution has brought about, but impossible to put them
into an acceptable chronological order, to show which came first and
which followed. The only hints that we get in this are the contrasting
facts that changes found to be common to several orders are presumably
the older, and those limited to fewer orders are to be taken as more
recent. However, the kinds of change provide some evidence as to the
related problem of the origin of Chelicerata and of Arachnida, and an
attempt may be made to summarize the whole story. It must, however,
be made with a prefatory note of caution.
First there may have arisen among the ancestral Arthropoda an
animal which may be described as a proto-chelicerate. It differed from
its contemporaries in three ways: its mouth, originally terminal as in
Annelida today, had moved to the ventral surface; the opisthosoma had
become limited to 12 somites, each with its separate tergite and sternite;
and thirdly there was a reduction of the middle portion or deutocerebrum
of the cerebral ganglion, so that the "brain" appeared to have a
reduced capacity.
These earliest Chcliccrata were marine organisms, and from them as a
starting point evolution seems to have proceeded in four directions. In
one of these an extraordinary reduction of the opisthosoma took place,
leaving little more than a plain sac, with few functions beyond a temporary
retention of the contents of the hind-gut. These became the
Pycnogonida.
Some Chelicerata developed in a less spectacular manner with
changes in the shape of the body and a modification of the last pair of
legs into paddles, useful for swimming. There was also a doublure or
folding down of the fore-edge of the carapace, which had the effect of
pushing the mouth back until it lay between the first pair oflegs. These
animals were conspicuously successful; they grew to an immense size,
and their numerous fossil remains are known to us today as Eurypterida.
An allied group never took so determinedly to swimming, but crawled
about on the mud and sand, helped by a telson in the form of a spine.
They survived to remain with us as a small group of Merostomata
known as the king-crabs.
11. PHYLOGENY: EVOLUTION 101
The really dramatic step was taken by some littoral Eurypterida
which began to leave the shallow water and to crawl on to the land.
The consequence was the evolution of the earliest Arachnida, a
terrestrial branch of Chelicerata facing all the problems oflife in dry air.
Their conspicuous need was for a new method of respiration, and it
seems to be very probable that if the book-gills of their ancestors had
not been pre-adapted for conversion into book-lungs, Arachnida as we
know them would never have become a possibility.
When once this step was taken, other changes followed. The prosomatic
tergites fused into a more efficient carapace, strengthening the
prosoma for its task of supporting eight legs. Segmentation remained,
however, in the opisthosoma and has disappeared only in the highest
groups. The posterior appendages vanished and the genital orifice
stabilized its position on the second opisthosomatic somite (Bristowe,
1958) 0
Later division into the orders now recognized was determined by the
production of a pedicel and by various changes in the mouth parts.
A view of arachnid evolution, wholly different from any of the above,
has been given, at length, by Firstman ( 1973). He investigated the
development and relations of the arterial system and the endosternite
in many representative genera, and came to the conclusion that a
relationship between these two systems is a primitive feature of the
Chelicerata. From this there followed an emphasis on the contrast
between the orders of Arachnida with book-lungs and those with
tracheae only.
He concluded with a scheme involving neoteny, followed by adaptive
radiation, which may be summarized thus:
(i) The ancestral scorpion was a neotenic Eurypterid, as is shown by
the development of the lungs, limbs, lateral eyes, endosternite and
arteries.
(ii) From this arose by adaptive radiation (a) modern scorpions (b)
other pulmonate Arachnida.
(iii) Apulmonate Arachnida were neotenic scorpions, as is shown by
the cephalothoracic tergites and sternites, limbs, respiratory organs,
endosternite and abdominal tagmata, and they divide into (a) Palpigradi
(b) other apulmonates.
Firstman's views are original and, backed as they are by much morphological
analysis, deserve careful appraisal.
In conclusion, an attempt may be made to bring together the different
ideas that have been expressed by students of arachnid phylogeny,
and to form them into a single picture.
The first point that emerges clearly enough is the separation of the
scorpions from the other orders. First on the scene, they have from the