_Grade A. ANOMOMERISTICA._
Sub-Class. TRILOBITAE.
Orders. Not satisfactorily determined.
_Grade B. NOMOMERISTICA._
Sub-Class I. PANTOPODA.
Order 1. Nymphonomorpha.
" 2. Ascorhynchomorpha.
" 3. Pycnogonomorpha.
Sub-Class II. EU-ARACHNIDA.
Grade a. DELOBRANCHIA, Lankester (_vel_ HYDROPNEUSTEA, Pocock).
Order 1. Xiphosura.
" 2. Gigantostraca.
Grade b. EMBOLOBRANCHIA, Lankester (_vel_ AEROPNEUSTEA, Pocock).
_Section_ [alpha]. _Pectinifera._
Order 1. Scorpionidea.
Sub-order a. Apoxypoda.
" b. Dionychopoda.
_Section_ [beta]. _Epectinata._
Order 2. Pedipalpi.
Sub-order a. Uropygi.
Tribe 1. Urotricha.
" 2. Tartarides.
Sub-order b. Amblypygi.
Order 3. Araneae.
Sub-order a. Mesothelae.
" b. Opisthothelae.
Tribe 1. Mygalomorphae.
" 2. Arachnomorphae.
Order 4. Palpigradi (= Microthelyphonidae).
Order 5. Solifugae (= Mycetophorae).
Order 6. Pseudoscorpiones (= Chelonethi).
Sub-order a. Panctenodactyli.
" b. Hemirtenodactyli.
Order 7. Podogona (= Ricinulel).
Order 8. Opiliones.
Sub-order a. Laniatores.
" b. Palpatores.
" c. Anepignathi.
Order 9. Rhynchostomi (= Acari).
Sub-order a. Notostigmata.
" b. Cryptostigmata.
" c. Metastigmata.
" d. Prostigmata.
" e. Astigmata.
" f. Vermiformia.
" g. Tetrapoda.
CLASS. ARACHNIDA.--Euarthropoda having two prosthomeres (somites which
have passed from a post-oral to a prae-oral position), the appendages
of the first represented by eyes, of the second by solitary rami which
are rarely antenniform, more usually chelate. A tendency is exhibited
to the formation of a metasomatic as well as a prosomatic carapace by
fusion of the tergal surfaces of the somites. Intermediate somites
forming a mesosoma occur, but tend to fuse superficially with the
metasomatic carapace or to become co-ordinated with the somites of the
metasoma, whether fused or distinct to form one region, the
opisthosoma (abdomen of authors). In the most highly developed forms
the two anterior divisions (tagmata) of the body, prosoma and
mesosoma, each exhibit six pairs of limbs, pediform and plate-like
respectively, whilst the metasoma consists of six limbless somites and
a post-anal spine. The genital apertures are placed in the first
somite following the prosoma, excepting where a praegenital somite,
usually suppressed, is retained. Little is known of the form of the
appendages in the lowest archaic Arachnida, but the tendency of those
of the prosomatic somites has been (as in the Crustacea) to pass from
a generalized bi-ramose or multi-ramose form to that of uni-ramose
antennae, chelae and walking legs.
The Arachnida are divisible into two grades of structure--according to
the fixity or non-fixity of the number of somites building up the
body:--
_Grade A_ (_of the Arachnida_). _ANOMOMERISTICA._--Extinct archaic
Arachnida, in which (as in the Entomostracous Crustacea) the number of
well-developed somites may be more or less than eighteen and may be
grouped only as head (prosoma) and trunk or may be further
differentiated. A telsonic tergal shield of greater or less size is
always present, which may be imperfectly divided into well-marked but
immovable tergites indicating incompletely differentiated somites. The
single pair of palpiform appendages in front of the mouth has been
found in one instance to be antenniform, whilst the numerous post-oral
appendages in the same genus were bi-ramose. The position of the
genital apertures is not known. Compound lateral eyes present; median
eyes wanting. The body and head have the two pleural regions of each
somite flattened and expanded on either side of the true gut-holding
body-axis. Hence the name of the sub-class signifying tri-lobed, a
condition realized also in the Xiphosurous Arachnids. The members of
this group, whilst resembling the lower Crustacea (as all lower groups
of a branching genealogical tree must do), differ from them
essentially in that the head exhibits only one prosthomere (in
addition to the eye-bearing prosthomere) with palpiform appendages (as
in all Arachnida) instead of two. The Anomomeristic Arachnida form a
single sub-class, of which only imperfect fossil remains are known.
Sub-class (of the Anomomeristica). TRILOBITAE.--The single sub-class
Trilobitae constitutes the grade Anomomeristica. It has been variously
divided into orders by a number of writers. The greater or less
evolution and specialization of the metasomatic carapace appears to be
the most important basis for classification--but this has not been
made use of in the latest attempts at drawing up a system of the
Trilobites. The form of the middle and lateral regions of the
prosomatic shield has been used, and an excessive importance attached
to the demarcation of certain areas in that structure. Sutures are
stated to mark off some of these pieces, but in the proper sense of
that term as applied to the skeletal structures of the Vertebrata, no
sutures exist in the chitinous cuticle of Arthropoda. That any partial
fusion of originally distinct chitinous plates takes place in the
cephalic shield of Trilobites, comparable to the partial fusion of
bony pieces by suture in Vertebrata, is a suggestion contrary to fact.
The Trilobites are known only as fossils, mostly Silurian and
prae-Silurian; a few are found in Carboniferous and Permian strata. As
many as two thousand species are known. Genera with small metasomatic
carapace, consisting of three to six fused segments distinctly marked
though not separated by soft membrane, are _Harpes_, _Paradoxides_ and
_Triarthrus_ (fig. 34). In _Calymene_, _Homalonotus_ and _Phacops_
(fig. 38) from six to sixteen segments are clearly marked by ridges
and grooves in the metasomatic tagma, whilst in _Illaenus_ the shield
so formed is large but no somites are marked out on its surface. In
this genus ten free somites (mesosoma) occur between the prosomatic
and metasomatic carapaces. _Asaphus_ and _Megalaspis_ (fig. 39) are
similarly constituted. In _Agnostus_ (fig. 40) the anterior and
posterior carapaces constitute almost the entire body, the two
carapaces being connected by a mid-region of only two free somites. It
has been held that the forms with a small number of somites marked in
the posterior carapace and numerous free somites between the anterior
and posterior carapace, must be considered as anterior to those in
which a great number of posterior somites are traceable in the
metasomatic carapace, and that those in which the traces of distinct
somites in the posterior or metasomatic carapace are most completely
absent must be regarded as derived from those in which somites are
well marked in the posterior carapace and similar in appearance to
the free somites. The genus _Agnostus_, which belongs to the last
category, occurs abundantly in Cambrian strata and is one of the
earliest forms known. This would lead to the supposition that the
great development of metasomatic carapace is a primitive and not a
late character, were it not for the fact that _Paradoxides_ and
_Atops_, with an inconspicuous telsonic carapace and numerous free
somites, are also Cambrian in age, the latter indeed anterior in
horizon to _Agnostus_.
[Illustration: FIG. 34.--Restoration of _Triarthrus Becki_, Green, as
determined by Beecher from specimens obtained from the Utica Slates
(Ordovician), New York. A, dorsal; B, ventral surface. In the latter
the single pair of antennae springing up from each side of the
camerostome or hypostome or upper lip-lobe are seen. Four pairs of
appendages besides these are seen to belong to the cephalic tergum.
All the appendages are pediform and bi-ramose; all have a prominent
gnathobase, and in all the exopodite carries a comb-like series of
secondary processes.
(After Beecher, from Zittel.) ]
On the other hand, it may well be doubted whether the pygidial or
posterior carapace is primarily due to a fusion of the tergites of
somites which were previously movable and well developed. The
posterior carapace of the Trilobites and of _Limulus_ is probably
enough in origin a telsonic carapace--that is to say, is the tergum of
the last segment of the body which carries the anus. From the front of
this region new segments are produced in the first instance, and are
added during growth to the existing series. This telson may enlarge,
it may possibly even become internally and sternally developed as
partially separate somites, and the tergum may remain without trace of
somite formation, or, as appears to be the case in _Limulus_, the
telson gives rise to a few well-marked somites (mesosoma and two
others) and then enlarges without further trace of segmentation,
whilst the chitinous integument which develops in increasing thickness
on the terga as growth advances welds together the unsegmented telson
and the somites in front of it, which were previously marked by
separate tergal thickenings. It must always be remembered that we are
liable (especially in the case of fossilized integuments) to attach an
unwarranted interpretation to the mere discontinuity or continuity of
the thickened plates of chitinous cuticle on the back of an Arthropod.
These plates may fuse, and yet the somites to which they belong may
remain distinct, and each have its pair of appendages well developed.
On the other hand, an unusually large tergal plate, whether terminal
or in the series, is not always due to fusion of the dorsal plates of
once-separate somites, but is often a case of growth and enlargement
of a single somite without formation of any trace of a new somite. For
the literature of Trilobites see (22*).
[Illustration: FIG. 35.--_Triarthrus Becki_, Green. a, Restored
thoracic limbs in transverse section of the animal; b, section across
a posterior somite; c, section across one of the sub-terminal somites.
(After Beecher.)]
[Illustration: FIG. 36.--_Triarthrus Becki_, Green. Dorsal view of
second thoracic leg with and without setae. en, Inner ramus; ex, Outer
ramus.
(After Beecher.)]
[Illustration: FIG. 37.--_Deiphon Forbesii_, Barr. One of the
Cheiruridae. Silurian Bohemia.
(From Zittel's Palaeontology.)]
[Illustration: FIG. 38.--_Dalmanites Kmulurus_, Green. One of the
_Phacopidae_, from the Silurian, New York.
(From Zittel.)]
[Illustration: FIG. 39.--_Megalaspis extenuatus_. One of the
_Asaphidae_ allied to _Illaenus_, from the Ordovician of East
Gothland, Sweden.
(From Zittel.)]
_Grade B (of the Aracknida) NOMOMERISTICA._--Arachnida in which,
excluding from consideration the eye-bearing prosthomere, the somites
are primarily (that is to say, in the common ancestor of the grade)
grouped in three regions of six--(a) the "prosoma" with palpiform
appendages, (b) the "mesosoma" with plate-like appendages, and (c) the
"metasoma" with suppressed appendages. A somite placed between the
prosoma and mesosoma --the prae-genital somite--appears to have
belonged originally to the prosomatic series (which with its ocular
prosthomere and palpiform limbs [Pantopoda], would thus consist of
eight somites), but to have been gradually reduced. In living
Arachnids, excepting the Pantopoda, it is either fused (with loss of
its appendages) with the prosoma (_Limulus_,[7] _Scorpio_), after
embryonic appearance, or is retained as a rudimentary, separate,
detached somite in front of the mesosoma, or disappears altogether
(excalation). The atrophy and total disappearance of ancestrally
well-marked somites frequently take place (as in all Arthropoda) at
the posterior extremity of the body, whilst excalation of somites may
occur at the constricted areas which often separate adjacent
"regions," though there are very few instances in which it has been
recognized. Concentration of the organ-systems by fusion of
neighbouring regions (prosoma, mesosoma, metasoma), previously
distinct, has frequently occurred, together with obliteration of the
muscular and chitinous structures indicative of distinct somites. This
concentration and obliteration of somites, often accompanied by
dislocation of important segmental structures (such as appendages and
nerve-ganglia), may lead to highly developed specialization
(individuation, H. Spencer), as in the Araneae and Opiliones, and, on
the other hand, may terminate in simplification and degeneration, as
in the Acari.
[Illustration: FIG. 40.--Four stages in the development of the
trilobite _Agnostus nudus_. A, Youngest stage with no mesosomatic
somites; B and C, stages with two mesosomatic somites between the
prosomatic and telsonic carapaces; D, adult condition, still with only
two free mesosomatic somites.
(From Korschelt and Heider.)]
[Illustration: FIG. 41.--Five Stages in the development of the
trilobite _Sao hirsuta_.
From Korschelt and Heider, after Barrande.
A, Youngest stage.
B, Older stage with distinct pygidial carapace.
C, Stage with two free mesosomatic somites between the prosomatic
and telsonic carapaces.
D, Stace with seven free intermediate somites.
E, Stave with twelve free somites; the telsonic carapace has not
increased in size.
a, Lateral eye.
g, So-called facial "suture" (not really a suture).
p, Telsonic carapace.]
The most important general change which has affected the structure of
the nomomeristic Arachnida in the course of their historic development
is the transition from an aquatic to a terrestrial life. This has been
accompanied by the conversion of the lamelliform gill-plates into
lamelliform lung-plates, and later the development from the
lung-chambers, and at independent sites, of tracheae or air-tubes (by
adaptation of the vasifactive tissue of the blood-vessels) similar to
those independently developed in _Peripatus_, Diplopoda, Hexapoda and
Chilopoda. Probably tracheae have developed independently by the same
process in several groups of tracheate Arachnids. The nomomeristic
Arachnids comprise two sub-classes--one a very small degenerate
offshoot from early ancestors; the other, the great bulk of the class.
[Illustration: FIG. 42.--So-called "trilobite stage" of _Limulus
polyphemus_. A, Dorsal; B, ventral view.
(from Korschelt and Heider, after Leuckart.)]
Sub-Class I. (of the Nomomeristica). PANTOPODA.--Nomomeristic
Arachnids, in which the somites corresponding to mesosoma and metasoma
have entirely aborted. The seventh, and sometimes the eighth,
leg-bearing somite is present and has its leg-like appendages fully
developed. Monomeniscous eyes with a double (really triple) cell-layer
formed by invagination, as in the Eu-arachnida, are present The
Pantopoda stand in the same relation to _Limulus_ and _Scorpio_ that
_Cyamus_ holds to the thoracostracous Crustacea. The reduction of the
organism to seven leg-bearing somites, of which the first pair, as in
so many Eu-arachnida, are chelate, is a form of degeneration connected
with a peculiar quasi-parasitic habit resembling that of the
crustacean Laemodipoda. The genital pores are situate at the base of
the 7th pair of limbs, and may be repeated on the 4th, 5th, and 6th.
In all known Pantopoda the size of the body is quite minute as
compared with that of the limbs: the alimentary canal sends a long
caecum into each leg (cf. the Araneae) and the genital products are
developed in gonocoels also placed in the legs.
[Illustration: FIG. 43.--One of the Nymphonomorphous Pantopoda,
_Nymphon hispidum_, showing the seven pairs of appendages 1 to 7; ab,
the rudimentary opisthosoma; s, the mouth-bearing proboscis.