Savory - Arachnida 1977

4 1. PROLEGOMENA
Present conventional classification divides the most successful groups
into a crustacean-myriapod-insect moiety and a eurypterid-arachnidpycnogonid
moiety, which seem to be reasonably distinguished as the
sub-phyla Antennata (or Mandibulata) and Chelicerata. This implies
that they arc the descendants of a common ancestor, while somewhere
on the fringe, as it were, stand the Onychophora, the Tardigrada or
water-bears, and the Pentastomida or Linguatulida. The relationships
between these three groups and the rest are not obvious.
The Onychophora, which include a score or so of species ofPeripatus
and its allies, were at one time thought to form a link between the
Annelida and the Arthropoda, both of which they resemble to some
extent. There is, however, reason to believe that they are neither the
descendants of the Annelida nor the ancestors of the Arthropoda. Less
easily placed in a simple and satisfying scheme of classification are the
Tardigrada and Pentastomida.
The opinions of Man ton ( 1973) can never be neglected. After an
exhaustive study of the basic structure of the limbs and mandibles, combined
with precise observation of the actions involved in moving and
feeding, she can avoid the risk of assuming "functionally impossible
ancestral stages" and suggests that the Arthropoda be arranged in three
phyla. These are the Crustacea, the Chelicerata and the U niramia. In
the last, which includes the Onychophora, the Myriapoda and the
Hexapoda, the limbs are fundamentally uniramous, contrasting in this
respect with the basically biramous limbs of the Crustacea and Chelicerata.
Complete acceptance of these views would result in the following
tabular summary:
Super-phylum Arthropoda
Phylum Trilobita
Phylum Crustacea
Phylum Uniramia
Class Onychophora
Class Myriapoda
Class Hexapoda
Phylum Chelicerata
Class Pycnogonida
Class Eurypterida
Class Xiphosura
Class Arachnida
The Chelicerata, with which alone this book is concerned, may at this
point be characterized as follows:
Arthropoda with a prosoma of six somites and an opisthosoma of 12
somites. The acron is not separable from the first somite, which carries one
l. THE PHYLUM ARTHROPODA
pair of pre-oml appendages, the chelicerae, primitively chelate organs.
There are Jive pairs of post-oral appendages, the first of which, the pedipalpi,
may be indistinguishable from the legs which follow it, or may be
specialized for certain Junctions.
In the opisthosoma the first somite may be reduced or may be absent. The
genital orifice always opens on the second somite. The sexes are always
separate. There is, in some orders, a visible distinction between a mesosoma
of seven and a metasoma of five somites.
Chelicemta may be aquatic, terrestrial or secondarily aquatic, respiration
being by gills, book-lungs or tracheae.
The nature of the relationship between the Chelicerata and the other
groups of Arthropoda has long been difficult to define. The suggestions
put forward in the past, though based as they have been on careful
research and considered opinion, have differed in an extraordinary way
from one another.
At one extreme stands the early theory of Savigny ( 1816), that
Arthropoda may be looked upon as headless Crustacea, that is to say
that from a common ancestor one class has retained, while the other has
lost, the anterior or cephalic somites. This implies that the chelicerae
and pedipalpi of Arachnida are modified legs. At the other extreme are
the hypotheses of Borner ( 1921) and Henriksen ( 1918), who regarded
the first three pairs oflegs of Arachnida as the mandibles and maxillae of
Mandibulata, and so picture Arachnida as walking on their mouth parts.
The consensus of modern opinion depends on the apparent homology
between the chelicerae of Chelicerata and the second antennae of
Mandibulata. A comparison between the chief classes of Arthropoda
then takes the following form:
MEROSTOMATA ARACHNIDA CRUSTACEA INSECTA
I. Antennae i Antennae
2. Chelicerae Chelicerae Antennae ii
3. Legs i Pedipalpi Mandibles Mandibles
4. Legs ii Legs i Maxillae i Maxillae
5. Legs iii Legs ii Maxillae ii Labium
6. Legs iv Legs iii Maxillepedes i Legs i
7. Legs v Legs iv Maxillipedes ii Legs ii
Legs iii
Too much significance should not be attached to the above table,
which is founded on the assumption of comparable rates and methods of
evolution in the classes concerned, while the bulk of the evidence is
against rather than in favour of such a correspondence.
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