Savory - Arachnida 1977

102 II. DE ARACHNIDIS
start preserved a form that has remained almost unchanged throughout
the ages. In this consistency they recall the tortoises, a reptilian order
with an equally unchanging equipment of genes.
For the rest, the outline of the story seems to read as follows.
Some newly-emerged immigrants were small enough to enable them
to absorb enough oxygen for survival if their bodies remained wet. This
occurred if they assumed a strictly cryptozoic mode of life, establishing
themselves in constantly dark and damp surroundings. Such were the
Palpigradi, Schizomida, Cyphophthalmi and Pseudoscorpiones (or the
earlier, ancestral forms of these orders).
The appearance of tracheae not only facilitated the business of
respiration, it was evidence of spontaneous or fortuitous changes in the
gene pool of the pioneers. There followed a tendency to increase in
size, which made possible, or perhaps necessary, an escape from the
cryptosphere, and adaptive radiation into habitats of more variable
natures. While the Palpigradi have, like the Scorpiones, retained a large
proportion of primitive characteristics, the ancestral Schizomida gave
rise to the early Uropygi, the Cyphophthalmi to the first Opiliones,
and the Pseudoscorpiones to the Solifugae.
The next step was the abandonment of the spermatophore as a device
for the impregnation of the females. In the ftagellum-bearing group
there came the loss of the ftagellum, converting "Uropygi" into
"Amblypygi", and finally, in the Araneae, the use of the male palp
for fertilization.
The descendants of the Cyphophthalmi were, on the one hand, the
Acari, on the other the Opiliones and the Ricinulei. The former developed
their own peculiar type of penis, while the latter made use of their
third legs to carry out the insertion of the sperm-packet. Lastly, the
Solifugae used the chelicerae for the same essential purpose.
Contemporary with these changes in adaptive radiation, there have
been, inevitably, many other specializations, such as the evolution of
courtship, the spinning of webs and the secretion of pheromones.
Nevertheless, in the three groups or sub-classes the course of evolution
has taken roughly parallel courses, which might be diagrammatically
represented thus:
Proscorp1us
j
Palpigradi Sch1zom1da pro- Cyphophthalml Pseudoscorp1ones
/ ~
Uropyg1 Amblypyg1 Cyphophthalml
t
""'
t
t ~
Scorp1ones Palp1grad1 Araneae A cor~ Op1hones R1c1nule1 Sol1fugae
t
l
12
Taxonomy: Classification
It is only to be expected that the difficulties of determining the evolutionary
history of the Arachnida will be reflected in the diversity of the
schemes which have been proposed for their classification.
The earliest systems need not be considered here; the first that will be
mentioned is that proposed by Ray Lankester in 1905, included in the
tenth edition of the "Encyclopaedia Britannica", and widely adopted
for some years. In this scheme the class was divided unequally into two
grades, Anomomeristica and Nomomeristica, depending on constancy
or inconstancy in the number of body somites. The former grade included
only the fossil Trilobita. The Nomomeristica were also unequally
divided into a sub-class Pantopoda, containing three orders of Pycnogonida,
and a sub-class Euarachnida, containing the rest. In Euarachnida
there were again two unequal divisions, the grade, Delobranchia or
Hydropneustea for Limulus and the fossil Eurypterida, and Embolobranchia
or Aeropneustea for the rest. Lastly, in Embolobranchia there
were distinguished the section Pectinifera for the order Scorpiones and
the section Epectinata for the remainder.
In 1904 Hansen and S0rensen suggested that the Pedipalpi, Araneida,
Palpigradi and Ricinulei should be grouped together as Arachnida
micrura, leaving the others in a second group, for which no name was
proposed.
Lankester's method of attacking the class by successively cutting off
the most aberrant group was preferred and in the circumstances was the
best way in which sub-divisions could be produced-if such subdivisions
were to be regarded from the start as necessary or desirable.
The method used by J. H. Comstock ( 1944) is noteworthy only because
it placed the Xiphosura in a separate but closely allied class, the
Palaeostraca.
Lankester's system was retained with modifications in a classification
offered by R. I. Pocock (1911). In thisschemeAnomomeristicaandNomomeristica
were preserved but the division of the latter into Pantopoda