ps, Muscular suctorial enlargement of the pharynx.
sal, Prosomatic pair of gastric caeca in Scorpio, called salivary
glands by some writers.
c^1, and c^2, The anterior two pairs of gastric caeca and ducts of
the mesosomatic region.
c^3, c^4 and c^5. Caeca and ducts of Scorpio not represented in
Limulus.
M, The Malpighian or renal caecal diverticula of Scorpio.
pro, The proctodaeum or portion of gut leading to anus and formed
embryologically by an inversion of the epiblast at that orifice.]
When it is admitted--as seems to be reasonable--that the primitive
Arachnida would, like the primitive Crustacea, be anomomeristic and
anomotagmic, we shall not demand of claimants for the rank of primitive
Arachnids agreement with Limulus and Scorpio in respect of the exact
number of their somites and the exact grouping of those somites; and
when we see how diverse are the modifications of the branches of the
appendages both in Arachnida and in other classes of Arthropoda (q.v.),
we shall not over-estimate a difference in the form of this or that
appendage exhibited by the claimant as compared with the higher
Arachnids. With those considerations in mind, the claim of the extinct
group of the trilobites to be considered as representatives of the lower
and more primitive steps in the Arachnidan genealogy must, it seems,
receive a favourable judgment. They differ from the Crustacea in that
they have only a single pair of prae-oral appendages, the second pair
being definitely developed as mandibles. This fact renders their
association with the Crustacea impossible, if classification is to be
the expression of genetic affinity inferred from structural coincidence.
On the contrary, this particular point is one in which they agree with
the higher Arachnida. But little is known of the structure of these
extinct animals; we are therefore compelled to deal with such special
points of resemblance and difference as their remains still exhibit.
They had lateral eyes[5] which resemble no known eyes so closely as the
lateral eyes of Limulus. The general form and structure of their
prosomatic carapace are in many striking features identical with that of
Limulus. The trilobation of the head and body--due to the expansion and
flattening of the sides or "pleura" of the tegumentary skeleton--is so
closely repeated in the young of Limulus that the latter has been called
"the trilobite stage" of Limulus (fig. 42 compared with fig. 41). No
Crustacean exhibits this trilobite form. But most important of the
evidences presented by the trilobites of affinity with Limulus, and
therefore with the Arachnida, is the tendency less marked in some,
strongly carried out in others, to form a pygidial or telsonic shield--a
fusion of the posterior somites of the body, which is precisely
identical in character with the metasomatic carapace of Limulus. When to
this is added the fact that a post-anal spine is developed to a large
size in some trilobites (fig. 38), like that of Limulus and Scorpio, and
that lateral spines on the pleura of the somites are frequent as in
Limulus, and that neither metasomatic fusion of somites nor post-anal
spine, nor lateral pleural spines are found in any Crustacean, nor all
three together in any Arthropod besides the trilobites and Limulus--the
claim of the trilobites to be considered as representing one order of a
lower grade of Arachnida, comparable to the grade Entomostraca of the
Crustacea, seems to be established.
The fact that the single pair of prae-oral appendages of trilobites,
known only as yet in one genus, is in that particular case a pair of
uni-ramose antennae--does not render the association of trilobites and
Arachnids improbable. Although the prae-oral pair of appendages in the
higher Arachnida is usually chelate, it is not always so; in spiders it
is not so; nor in many Acari. The bi-ramose structure of the post-oral
limbs, demonstrated by Beecher in the trilobite Triarthrus, is no more
inconsistent with its claim to be a primitive Arachnid than is the
foliaceous modification of the limbs in Phyllopods inconsistent with
their relationship to the Arthrostracous Crustaceans such as Gammarus
and Oniscus.
Thus, then, it seems that we have in the trilobites the representatives
of the lower phases of the Arachnidan pedigree. The simple anomomeristic
trilobite, with its equi-formal somites and equi-formal appendages, is
one term of the series which ends in the even more simple but degenerate
Acari. Between the two and at the highest point of the arc, so far as
morphological differentiation is concerned, stands the scorpion; near to
it in the trilobite's direction (that is, on the ascending side) are
Limulus and the Eurypterines--with a long gap, due to obliteration of
the record, separating them from the trilobite. On the other
side--tending downwards from the scorpion towards the Acari--are the
Pedipalpi, the spiders, the book-scorpions, the harvest-men and the
water-mites.
The strange nobody-crabs or Pycnogonids occupy a place on the ascending
half of the arc below the Eurypterines and Limulus. They are strangely
modified and degenerate, but seem to be (as explained in the systematic
review) the remnant of an Arachnidan group holding the same relation to
the scorpions which the Laemodipoda hold to the Podophthalmate
Crustacea.
* * * * *
We have now to offer a classification of the Arachnida and to pass in
review the larger groups, with a brief statement of their structural
characteristics.
In the bibliography at the close of this article (referred to by leaded
arabic numerals in brackets throughout these pages), the titles of works
are given which contain detailed information as to the genera and
species of each order or sub-order, their geographical distribution and
their habits and economy so far as they have been ascertained. The
limits of space do not permit of a fuller treatment of those matters
here.