Savory - Arachnida 1977

6 I PROLEGOMENA
The comparison implied loses much of its value if the Arthropoda
are not regarded as a monophyletic group. As the evidence for its
polyphyletic nature becomes steadily more convincing, the possibility
of accurately defining analogies between all the somites and appendages
of the head and thorax becomes proportionately more difficult.
This leads to a further point. In the body of an insect there is a
clear distinction between the head and the thorax, the latter name being
given to the three somites that carry the three pairs of legs. The head
of an arachnid is not so easily defined. A reasonable view is that not
more than one somite can be called cephalic, and its distinction from
the thorax is oflittle significance.
2
The Class Arachnida
Chelicerate Arthropoda, in which the adult body is fundamentally composed
of 18 somites, divisible into a prosoma of six and an opisthosoma of
12 units. Segmentation may be obscured in either or both of these tagmala
by the fusion or suppression of the sclerites. There are not more
than 12 simple ocelli. The prosoma carries six pairs of appendages, the
chelicerae, the pedipalpi and Jour pairs of legs. The chelicerae, in front of
the mouth, are of two or three pieces and are either chelate or unchelate. The
pedipalpi are of six podomeres, are either chelate weapons or leg-like limbs,
and often carry gnathobases on their coxae. The legs are of seven podomeres:
the anterior pair or pairs may carry gnathobases, the tarsi end in two or three
smooth or pectinate claws. The sternum may be visibly segmented or entire,
and is often reduced, hidden or obliterated by the encroachment of the
coxae. The opisthosoma may be united to the prosoma by a narrow pedicel,
the seventh somite, and is usually without appendages. The respiratory
system, consisting of book-lungs or tracheae or both, open on this part of the
body. The sexes are separate, with orifices on the lower side of the second
opisthosomatic somite. Courtship is usual and is often elaborate. The individuals
are generally terrestrial, carnivorous, nocturnal and cryptozoic.
Instinctive behaviour is highly developed and social organization is very rare.
Carolus Linnaeus, in "Systema Naturae", had given inadequate attention
to the invertebrates, and improvements to this part of his great
achievement could not long be delayed. The first biologist to undertake
this task was Lamarck.
Jean Baptiste de Monet, Chevalier de Lamarck (1744-1829), botanist
at theJardin du Roi in Paris, was elected at the age of 50 to be the
professor in charge of "Insectes et Vermes" at the newly-founded
Museum d'Histoire N aturelle, becoming thereby the first occupant of a
chair of invertebrate zoology. To him fell the task of creating a new
system of classification, which he performed with such genius that much
of his work has survived until today.