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297
ARACHNIDA

when by such an initial coincidence the two members have been particularized. The chances against these two selected members exhibiting another really independent homoplastic agreement are enormous: let us say 10,000 to 1. The chances against yet another coincidence are a hundred million to one, and against yet one more “coincidence” they are the square of a hundred million to one. Homoplasy can only be assumed when the coincidence is of a simple nature, and is such as may be reasonably supposed to have arisen by the action of like selective conditions upon like material in two separate lines of descent.[1]

So, too, degeneration is not to be lightly assumed as the explanation of a simplicity of structure. There is a very definite criterion of the simplicity due to degeneration, which can in most cases be applied. Degenerative simplicity is never uniformly distributed over all the structures of the organism. It affects many or nearly all the structures of the body, but leaves some, it may be only one, at a high level of elaboration and complexity. Ancestral simplicity is more uniform, and does not co-exist with specialization and elaboration of a single organ. Further: degeneration cannot be inferred safely by the examination of an isolated case; usually we obtain a series of forms indicating the steps of a change in structure—and what we have to decide is whether the movement has been from the simple to the more complex, or from the more complex to the simple. The feathers of a peacock afford a convenient example of primitive and degenerative simplicity. The highest point of elaboration in colour, pattern and form is shown by the great eye-painted tail feathers. From these we can pass by gradual transitions in two directions, viz. either to the simple lateral tail feathers with a few rami only, developed only on one side of the shaft and of uniform metallic coloration—or to the simple contour feathers of small size, with the usual symmetrical series of numerous rami right and left of the shaft and no remarkable colouring. The one-sided specialization and the peculiar metallic colouring of the lateral tail feathers mark them as the extreme terms of a degenerative series, whilst the symmetry, likeness of constituent parts inter se, and absence of specialized pigment, as well as the fact that they differ little from any average feather of birds in general, mark the contour feather as primitively simple, and as the starting-point from which the highly elaborated eye-painted tail feather has gradually evolved.

Applying these principles to the consideration of the Arachnida, we arrive at the conclusion that the smaller and simpler Arachnids are not the more primitive, but that the Acari or mites are, in fact, a degenerate group. This was maintained by Lankester in 1878 (19), again in 1881 (20); it was subsequently announced as a novelty by Claus in 1885 (21). Though the aquatic members of a class of animals are in some instances derived from terrestrial forms, the usual transition is from an aquatic ancestry to more recent land-living forms. There is no doubt, from a consideration of the facts of structure, that the aquatic water-breathing Arachnids, represented in the past by the Eurypterines and to-day by the sole survivor Limulus, have preceded the terrestrial air-breathing forms of that group. Hence we see at once that the better-known Arachnida form a series, leading from Limulus-like aquatic creatures through scorpions, spiders and harvest-men, to the degenerate Acari or mites. The spiders are specialized and reduced in apparent complexity, as compared with the scorpions, but they cannot be regarded as degenerate since the concentration of structure which occurs in them results in greater efficiency and power than are exhibited by the scorpion. The determination of the relative degree of perfection of organization attained by two animals compared is difficult when we introduce, as seems inevitable, the question of efficiency and power, and do not confine the question to the perfection of morphological development. We have no measure of the degree of power manifested by various animals—though it would be possible to arrive at some conclusions as to how that “power” should be estimated. It is not possible here to discuss that matter further. We must be content to point out that it seems that the spiders, the pedipalps, and other large Arachnids have not been derived from the scorpions directly, but have independently developed from aquatic ancestors, and from one of these independent groups—probably through the harvest-men from the spiders—the Acari have finally resulted.


1911 Britannica-Arachnida-Beck.png

After Beck, Trans. Zool. Soc. Vol. xi., 1883.


Fig. 31.—Diagram of a lateral view of a longitudinal section of a scorpion.

d, Chelicera. dam, Muscle from carapace to a
ch, Chela. praeoral entosclerite.
cam, Camerostome. ad, Muscle from carapace to
m, Mouth. entosternum.
ent, Entosternum. md, Muscle from tergite of genital
p, Pecten. somite to entosternum (same as
stig1, First pulmonary aperture. dpm in fig. 30).
stig4, Fourth pulmonary aperture.                 dv1 to dv6, Dorso-ventral muscles
(same as the series labelled tsm in
fig. 30).
pv1 to pv7, The seven veno-
pericardiac muscles of the right
side (labelled VPM in fig. 30).



1911 Britannica-Arachnida-Benham.png

After Benham, Trans. Zool. Soc. Vol. xi., 1883.
Fig. 32.—Diagram of a lateral view of a longitudinal section of Limulus.

Suc, Suctorial pharynx. br1 to br5, Branchial appendages,
al, Alimentary canal. met, Unsegmented metasoma.
Ph, Pharynx. Entap4, Fourth dorsal entapophysis of left
M, Mouth. side.
Est, Entosternum. tsm, Tergo-sternal muscles, six pairs as in
VS, Ventral venous sinus.                     Scorpio (labelled dv in fig. 31).
chi, Chilaria. VPM1 to VPM8, The eight pairs of veno-
go, Genital operculum. pericardiac muscles (labelled pv in fig.
31). VPM1 is probably represented in
Scorpio, though not marked in figs. 30
and 31.
  1. A great deal of superfluous hypothesis has lately been put forward in the name of “the principle of convergence of characters” by a certain school of palaeontologists. The horse is supposed by these writers to have originated by separate lines of descent in the Old World and the New, from five-toed ancestors! And the important consequences following from the demonstration of the identity in structure of Limulus and Scorpio are evaded by arbitrary and even phantastic invocations of a mysterious transcendental force which brings about “convergence” irrespective of heredity and selection. Morphology becomes a farce when such assumptions are made.  (E. R. L.)