ARACHNIDA—INTRODUCTION
The Arachnida, together with the Crustacea, Insecta, Myriapoda, and
_Peripatus_, make up the great phylum Arthropoda, a phylum which, from
the point of view of numbers of species and of individuals, is the
dominant one on this planet, and from the point of view of intelligence
and power of co-operating in the formation of social communities is
surpassed but by the Vertebrata. The Arachnida form a more diverse class
than the Insecta; they differ, perhaps, as much _inter se_ as do the
Crustacea, and in structure as in size and habit they cover a wide
range.
Lankester in his article upon the Arthropoda, in the tenth edition of
the _Encyclopaedia Britannica_, dwells upon the fact that whereas the
adult _Peripatus_ has but one persisting segment in front of the head,
and its mouth is between the second persisting appendages, in Arachnids
the mouth has receded and lies between the bases of the appendages
(pedipalpi) of the third persisting segment, while two of the persisting
segments, those of the eyes and chelicerae, have passed in front of the
mouth. This process has continued in the Crustacea and in the Insecta;
in both of these classes there are three embryonic segments in front of
the adult mouth, which lies between the appendages of the fourth
segment.
In the larger and more complex Arachnida the number of segments is fixed
and constant, and though possibly no adult member of the group, owing to
the suppression of one or more segments during the ontogeny, ever shows
the full number at any one time, the body can be analysed into
twenty-one segments. It is interesting to note that the same number of
segments occurs in Insecta and in the higher Crustacea.[204] The
significance of this fact is not perhaps apparent, but it seems to
indicate “a sort of general oneness, if I may be allowed to use so
strong an expression,” as Mr. Curdle said when discussing the unities of
the drama with Nicholas Nickleby.
These segments are arranged in higher categories or “tagmata,” of which
we can recognise three: (i.) the prosoma, (ii.) the mesosoma, and (iii.)
the metasoma. The prosoma, sometimes termed the “cephalothorax,”
includes all the segments in front of the genital pore. According to
this definition the prosoma includes the segment which bears the
chilaria in _Limulus_ (the King-crab) and the pregenital but evanescent
segment in Scorpions. The mesosoma begins with the segment bearing the
genital pore, and ends with the last segment which bears free
appendages, six segments in all. The metasoma also consists of six
segments which have no appendages; together with the mesosoma it forms
the abdomen of some writers. The anus lies posteriorly on the last
segment, and behind it comes in the higher forms a post-anal “telson,”
taking in Scorpions the form of the sting, in King-crabs that of the
spine.
As we have seen, it is only in the more typical and perhaps higher forms
that we can find our twenty-one segments, and then they are never
present all at once. In many groups of Arachnids the number is reduced
at the hinder end, and obscured by the fusion of neighbouring segments.
Also segments are dropped as a stitch is dropped when knitting; for
instance, in the rostral segment which has a neuromere, and in the
Spider _Trochosa_ vestigial antennae, or in Scorpions the pregenital
segment.
Primitive Arachnids appear to have lived in the sea and to have breathed
by gill-books borne on appendages; when their descendants took to living
on land and to breathing air instead of water, the gill-books sank into
the body and became lung-books, to which the air was admitted by
slit-like stigmata. In other terrestrial forms the lung-books are
replaced by tracheae which in their structure and arrangement resemble
those of _Peripatus_ rather than those of the Insecta. The circulation,
as is usual in Arthropods, is largely lacunar, but in Scorpions and
_Limulus_ the arteries form definite channels, and are in fact better
developed than in any other Arthropod.
As a rule the alimentary canal in Arachnids is no longer than the
distance between the mouth and the anus; but in the King-crab, where the
mouth is pushed back almost to the centre of the body, there is a
flexure in the median vertical plane. Paired glands, usually called the
liver, open into the mesenteron; food passes into the lumen of these
glands, and is probably digested there. In many Arachnids these glands
extend into the limbs. In those members of the group that have become
terrestrial the nitrogenous excreta are separated out by Malpighian
tubules which open into the proctodaeum; but coxal glands, homologous
with the green gland and shell-glands of Crustacea, may coexist, and in
the aquatic _Limulus_ these alone are found. They usually open on the
base of one or more pairs of walking legs.
The endosternite, or internal skeletal plate to which muscles are
attached, reaches a higher development in the Arachnida than in the
Crustacea. In Scorpions it forms a kind of diaphragm incompletely
separating the cavities of the pro- and meso-soma.
The supra-oesophageal ganglion supplies the two existing segments which
have slipped before the mouth, _i.e._ those of the eyes and of the
chelicerae. The post-oral ganglia in the Acarina, the Pedipalpi, the
Solifugae, and the Araneae have fused into a central nerve-mass, from
which nerves radiate; but in _Limulus_ the prosomatic appendages are all
supplied from the nerve-ring. The chief sense-organs are eyes of the
characteristic Arthropod type, and sensory hairs of a great variety of
complexity. Scorpions and Spiders have stridulating organs, and we may
assume that they have also some auditory apparatus; perhaps some of the
hairs just mentioned act as hearing organs.
Arachnids are male and female; they do not reproduce asexually, and
there is no satisfactory proof that they ever reproduce
parthenogenetically. As a rule there is little external difference
between the two sexes, except in Spiders, where the male is as a rule
smaller than the female, and when adult has the pedipalpi modified for
use in depositing the spermatophores. The ovaries and testes are
annular, and with their ducts encircle the alimentary canal in Mites and
Phalangids; in Scorpions and King-crabs they have become retiform.
Mites, Scorpions, and Pedipalps are viviparous, their eggs developing in
the ovary or in a uterus. Other Arachnids lay eggs, and many Spiders and
Pseudoscorpions carry their eggs about with them. As a rule the young is
but a miniature of the parent, and the Arachnid, unlike the Crustacean
or Insect, undergoes little or no metamorphosis.
A certain number of Mites are parasitic in plants and in animals, and a
few, together with a few Spiders, have resumed the aquatic life of their
remote ancestors. The members of some Orders, such as the Solifugae and
Opiliones, are nocturnal, and many are provided with silk-glands and
weave webs which reach their highest pitch of perfection amongst the
Spiders. At times—especially is this the case with the Mites—enormous
numbers of individuals live together, but they never show the least
adaptation to communal life, and no individuals are ever specialised to
perform certain functions, as is the rule in communities of social
Insects.
With the two exceptions that we regard the Trilobites as more nearly
allied to the Crustacea, and have therefore considered them apart, and
have treated the Pycnogonida independently of the Arachnida, we have
followed Lankester in his classification, though not always in his
nomenclature:—
Sub-class 1. Delobranchiata[205] (Merostomata).
Order (i.) Xiphosura.
Order (ii.) Eurypterida (= Gigantostraca, Extinct).
Sub-class 2. Embolobranchiata.
Order (i.) Scorpionidea.
Order (ii.) Pedipalpi.
Order (iii.) Araneae.
Order (iv.) Palpigradi.
Order (v.) Solifugae.
Order (vi.) Chernetidea (= Pseudoscorpiones).
Order (vii.) Podogona.
Order (viii.) Phalangidea (= Opiliones).
Order (ix.) Acarina.
APPENDICES
(i.) Tardigrada.
(ii.) Pentastomida.