Coxal glands have since been recognized and described in other Arachnida.
BOOK-SCORPION, or FALSE SCORPION, minute arachnids superficially resembling tailless scorpions and belonging to the order Pseudoscorpiones of the class Arachnida.
Carpenter (1899, 1902-1904) has lately endeavoured to show an exact numerical correspondence in segmentation between the Hexapoda, the Crustacea, the Arachnida, and the most primitive of the Diplopoda.
On either view it may be believed that the Hexapoda arose with the allied classes from a primitive arthropod stock, while the relationships of the class are with the Crustacea, the Chilopoda and the Diplopoda, rather than with the Arachnida.
SPIDERS, the common English name of Arachnida of the order Araneae, resembling the Pedipalpi in many structural points, but differing from them as well as from all other Arachnida in retaining short abdominal appendages known from their silk-manipulating function as spinnerets or spinning mamillae, with which are associated silk glands.
Spiders, in short, must be regarded as the most highly organized and the most successful members of the class Arachnida.
The possession of silk-glands has also profoundly influenced the geographical distribution of spiders and has enabled them to cross arms of the sea and establish themselves on isolated oceanic islands which most of the orders of Arachnida are unable to reach.
ARACHNIDA, the zoological name given in 1815 by Lamarck (Gr.
Lamarck included the Thysanura and the Myriapoda in his class Arachnida.
The position of the Arachnida in the great sub-phylum Arthropoda, according to recent anatomical and embryological researches, is explained in the article Arthropoda.
The Arachnida form a distinct class or line of descent in the grade Euarthropoda, diverging (perhaps in common at the start with the Crustacea) from primitive Euarthropods, which gave rise also to the separate lines of descent known as the classes Diplopoda, Crustacea, Chilopoda and Hexapoda.
There are a number of other important points of structure besides those referring to the somites and appendages in which Limulus agrees with Scorpio or other Arachnida and differs from other Arthro- '11'1 poda.
The Crustacea have, in fact, three prosthomeres in the head and the Arachnida only two, and Limulus agrees with the Arachnida in this respect and differs from the Crustacea.
The coxal glands of the Arachnida are structures of the same nature as the green glands of the higher Crustacea and the so-called " shell glands " of the Entomostraca.
On the other hand, the entosternite of the Arachnida is a very large and important feature FIG.
This arrangement has not hitherto been detected in any other class than the Arachnida, and if it should ultimately prove to be peculiar to that group, would have considerable weight as a proof of the close genetic affinity of Limulus and Scorpio.
The mouth is relatively smaller in Scorpio than in Limulus - in fact is minute, as it is in all the terrestrial Arachnida which suck the juices of either animals or plants.
It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcely been appreciated by the writers in question), really belongs to the Crustacean line of descent, whilst Scorpio, by possessing Malpighian tubes, is declared to be unmistakably tied together with the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes.
In other words, the Malpighian tubes of the terrestrial Arachnida are homoplastic with those of Hexapoda and Myriapoda, and not homogenetic with them.
We are compelled to take a similar view of the agreement between the tracheal air-tubes of Arachnida and other tracheate Arthropods.
Leaving that question for consideration in connexion with the systematic statement of the characters of the various groups of Arachnida which follows on p. 475, it is well now to consider the following question, viz., seeing that Limulus and Scorpio are such highly developed and specialized forms, and that they seem to constitute as it were the first and second steps in the series of recognized Arachnida - what do we know, or what are we led to suppose with regard to the more primitive Arachnida from which the Eurypterines and Limulus and Scorpio have sprung ?
On the contrary, this particular point is one in which they agree with the higher Arachnida.
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.
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.
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.
Tabular Classification 1 Of The Arachnida.
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).
- 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.
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.
Pocock accepts those views in all essential points and has, as a special student of the Arachnida, given to them valuable expansion and confirmation.
' Grade B (of the Arachnida) Nomomeristica.
- Arachnida in which, excluding from consideration the eye-bearing prosthomere, the somites are primarily (that is to say, in the common Q FIG.
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.
(of the Nomomeristic Arachnida).
All Arachnida, including Limulus, feed by suctorial action in essentially the same way as Scorpio.
It is probable that the Pedipalpi, Araneae, and Podogona have been separately evolved as distinct lines of descent from the ancient aquatic Arachnida.
- The Spiders are the most numerous and diversified group of the Arachnida; about 2000 species are known.
(From Lankester, " Limulus and Arachnid.") Galeodes has been made the means of a comparison between the structure of the Arachnida and Hexapod insects by Haeckel and other writers, and it was at one time suggested that there was a genetic affinity between the two groups - through Galeodes, or extinct forms similar to it.
J., " Organs and Characters in Different Orders of Arachnida," Entomol.
Hansen and Sorensen, The Order Palpigradi, Thorell (Koenenia), and its Relationships with other Arachnida," Ent.
Hansen and Sorensen, On Two Orders of Arachnida (Cambridge, 1904); Sorensen, " Opiliones laniatores," Nat.
General works on Arachnida: - Blanchard, " Les Arachnides " in L' Organisation du regne animal; Gaubert, " Recherches sur les Arachnides," Ann.
xiii., 1892; Koch, C., Die Arachniden (16 vols., Nuremberg, 1831-1848); Koch, Keyserling and Sorensen, Die Arachniden Australiens (Nuremberg, 1871-1890); Pocock, Arachnida of British India (London, 1900); Idem, " On African Arachnida," in Proc. Zool.
Hist., 1897-1900; Simon, Les Arachnides de la France (7 vols., Paris, 1874-1881); Thorell, "Arachnida from the Oriental Region," Ann.
As important original monographs we note-Az drapoly a Fiumei obolben (Ebb and Flow in the Gulf of Fiume), by Emil Stahlberger (1874); Magyarorszdg pokfaundja (The Arachnida of Hungary), by Otto Hermann (1876-1878); Magyarorszdg vaskovei es vastermenyei (The Iron Ores and 1 The translator of Macaulay.
Classes: Arachnida, Insecta (including Sub-Classes M y riapoda, Hexapoda), Crustacea (including Sub-Classes Entomostraca, Malacostraca), Epizoa (Epizootic Crustacea), Annellata (Chaeto p ods and Leeches), Cirripedia.
Orders: Myriapoda, Arachnida (Accra, Latr.), and Hexapoda.
Classes: Crustacea, Arachnida,Myriapoda,Insecta,Chaetognatha, Annelida.
TRILOBITES, extinct Arthropoda, formerly classified with the Crustacea, but of late years relegated to the Arachnida, which occurred abundantly in seas of the Cambrian and Silurian periods, but disappeared entirely at the close of the Palaeozoic epoch.
As in all Arachnida there is only a single pair of appendages in front of the mouth, and these were onebranched, long and filiform and acted as antennae.
Albinism is restricted to no particular class of the animal kingdom; for partial albinism at least is known to occur in Coelentera, worms, Crustacea, Myriapoda, Coleoptera,Arachnida and fishes.
APTERA (Greek for "wingless"), a term in zoological classification applied by Linnaeus to various groups of wingless arthropods, including some of the insects, the centipedes, the millipedes, the Arachnida (scorpions, spiders, &c.) and the Crustacea.
They are distributed thus: Vertebrata, 8 species; Insecta, 17; Arachnida, 12; Myriapoda, 2; Crustacea, 5; Vermes, 3; Mollusca, 1.
It is to be noted, however, that the Trilobita, which, according to the classification here adopted, are dealt with under Arachnida, are not very far removed, except in such characters as the absence of a shellfold and of eye-stalks, from the primitive Crustacean here sketched.
PEDIPALPI, Arachnida (q.v) related to the spiders, and serving in a measure to bridge over the structural interval between the latter and the scorpions.
A third division, the Tartarides, a subordinate group of the Uropygi, contains minute Arachnida differing principally from the typical Uropygi in having the caudal process unjointed and short.
Apart from the Tartarides, the Pedipalpi are large or medium-sized Arachnida, nocturnal in habits and spending the day under stones, logs of wood or loosened bark.
Aristotle had included in one class "Entoma" the six-legged arthropods which form the modern zoological class of the Hexapoda or Insecta, besides the Arachnida, the centipedes and the millipedes.
TICKS, the common name for Arachnida belonging to the order Acari, of which they constitute the two families, Ixodidae and Argasidae.
The names Condylopoda and Gnathopoda have been subsequently proposed for the same group. The word refers to the jointing of the chitinized exo-skeleton of the limbs or lateral appendages of the animals included, which are, roughly speaking, the Crustacea, Arachnida, Hexapoda (so-called " true insects "), Centipedes and Millipedes.
Lamarck's penetrating genius is chiefly responsible for the shrinkage of the word Insecta, since it was he who, forty years after Linnaeus's death, set up and named the two great classes Crustacea and Arachnida (included by Linnaeus under Insecta as the order " Aptera "), assigning to them equal rank with the remaining Insecta of Linnaeus, for which he proposed the very appropriate class-name " Hexapoda."
Lamarck, however, appears not to have insisted on this name Hexapoda, and so the class of Pterygote Hexapods came to retain the group-name Insecta, which is, historically or etymologically, no more appropriate to them than it is to the classes Crustacea and Arachnida.
as pairs in each ring-like segment or somite of the body, and some of these are in all cases retained as gonoducts and often as renal excretory organs (green glands, coxal glands of Arachnida, not crural glands, which are epidermal in origin); but true nephridia, genetically identical with the nephridia of earthworms, do not occur (on the subject of coelom, coelomoducts and nephridia, see the introductory chapter of part ii.
The Arachnida present the first stage of progress.
The Arachnida are therefore tritognathous.
The three prosthomeres or prae-oral somites of Crustacea due to the sinking back of the mouth one somite farther than in Arachnida are not clearly indicated by coelomic cavities in the embryo, but their existence is clearly established by the development and position of the appendages and by the neuromeres.
As in Crustacea and Arachnida, FIG.
In Arachnida the highest forms exhibit a fusion of the tergites of five post-oral somites to form one continuous carapace united with the terga of the two prosthomeres.
It is not true that all the biting processes of the Arthropod limb are thus produced - for instance, the jaws of Peripatus are formed by the axis or corm itself, whilst the poisonjaws of Chilopods, as also their maxillae, appear to be formed rather by the apex or terminal region of the ramus of the limb; but the opposing jaws (= hemignaths) of Crustacea, Arachnida and Hexapoda are gnathobases, and not the axis or corm.
Hexapoda and Arachnida); the penultimate joint with a process equal in length to the last joint, so as to form a nipping organ (chelae of Crustaceans and Arachnids); the last joint reflected and movable on the penultimate, as the blade of a clasp-knife on its handle (the retrovert, After Lankester, Q.
They show in the monomeniscous eye (see article Arachnida, fig.
23, article Arachnida) a single retinula or group of nerve-end-cells is grouped beneath each associated lens.
22, article Arachnida).
These eyes are called diplostichous, and occur in Arachnida and Hexapoda (fig.
24, article Arachnida).
We do not find them in any Arachnida.
But they seem to point to a community of origin of Hexapods and Crustacea in regard to the complicated ommatidia of the compound eye, and to a certain isolation of the Arachnida, which are, however, traceable, so far as the eyes are concerned, to a distant common origin with Crustacea and Hexapoda through the very simple compound eyes (monostichous, polymeniscous) of Limulus.
We are driven by the conclusions arrived at as to the derivation of the Arachnida from branchiate ancestors, independently of the other tracheate Arthropods, to formulate the conclusion that tracheae have been independently developed in the Arachnidan class.
in the Onychophora, in more than one group of Arachnida, in Diplopoda, and again in the Hexapoda and Chilopoda.
in Chilopoda, Diplopoda and Arachnida.
The absence of such renal caeca in Limulus and their presence in the terrestrial Arachnida is precisely on a parallel with their absence in aquatic Crustacea and their presence in the feebly branchiate Amphipoda.
A character of great diagnostic value in the more primitive Arachnida is the tendency of the chitinous investment of the tergal surface of the telson to unite during growth with that of the free somites in front of it, so as to form a pygidial shield or posterior carapace, often comprising as many as fifteen somites (Trilobites, Limulus).
The ancestral stock was (as in the Arachnida) pantognathobasic, that is to say, had a gnathobase or jaw-process on the base of every post-oral appendage.
These have small and insignificant rami, or none at all, a feature in which the Arachnida differ from them.
A characteristic, comparable in value to that presented by the pygidial shield of Arachnida, is the frequent development of a pair of long appendages by the penultimate somite, which with the telson form a trifid, or, when that is small, a bifid termination to the body.
The lateral eyes of Crustacea are polymeniscous, with highly specialized retinulae like those of Hexapoda, and unlike the simpler compound lateral eyes of lower Arachnida.
It appears from observation of the embryo that whilst the first prosthomere of Centipedes has its appendages reduced and represented only by eye-patches (as in Arachnida, Crustacea and Hexapoda), the second has a rudimentary antenna, which disappears, whilst the third carries the permanent antennae, which accordingly correspond to the second antennae of Crustacea, and are absent in Hexapoda.
The Hexapoda are not only all confined to a very definite disposition of the somites, appendages and apertures, as thus indicated, but in other characters also they present the specialization of a narrowly-limited highly-developed order of such a class as the Crustacea rather than a range from lower more generalized to higher more specialized forms such as that group and also the Arachnida present.
In other cases (some larvae) stigmata are absent; in other cases again a single stigma is developed, as in the smaller Arachnida and Chilopoda, in the median dorsal line or other unexpected position.
Our general conclusion from a survey of the Arthropoda amounts to this, that whilst Peripatus, the Diplopoda, and the Arachnida represent terrestrial offshoots from successive lower grades of primitive aquatic Arthropoda which are extinct, the Crustacea alone present a fairly full series of representatives leading upwards from unspecialized forms. The latter were not very far removed from the aquatic ancestors (Trilobites) of the Arachnida, but differed essentially from them by the higher specialization of the head.
It is probably owing to the possession of such glands and the varied purposes for which the silk is used that spiders as a group far surpass the other orders of Arachnida, with the possible exception of the Acari (mites and ticks), in diversity of form and of size, in numbers of genera and species, in extent of geographical distribution, and in adaptation to varied habitats.
The phenomena, known as "protective resemblance," or similarity to inanimate objects or vegetation, and the kindred phenomenon of "mimicry," or beneficial likeness to certain protected species of animals, are common in the group. In these particulars, considered in their entirety, spiders show a marked contrast to other Arachnida, such as the scorpions, pedipalps, book-scorpions and so-called harvest spiders, which by comparison are remarkably uniform, within the limits of the orders, in structure, habits and other respects.
These three authors definitely separated the Arachnida, Crustacea and Myriapoda as classes distinct from the Insecta (see Hexapoda).
PENTASTOMIDA, or Linguatulina, vermiform entoparasitic animals, of which the exact zoological position is unknown, although they are usually regarded as highly modified degenerate Arachnida of the order Acari.
- Modern views as to the classification and affinities of the Arachnida have been determined by the demonstration that Limulus and the extinct Eurypterines (Pterygotus, &c.) are Arachnida; that is to say, are identical in the structure and relation of so many important parts with Scorpio, whilst differing in those respects from other Arthropoda, that it is impossible to suppose that the identity is due to homoplasy or convergence, and the conclusion must be accepted that the resemblances arise from close genetic relationship. The view that Limulus, the king-crab, is an Arachnid was maintained as long ago as 182 9 by Strauss-Diirckheim (1), on the ground of its possession of an internal cartilaginous sternum - also possessed by the Arachnida (see figs.
- It appears (see Arthropoda) that there is embryological evidence of the existence of two somites in Arachnida which were originally post-oral, but have become prae-oral by adaptational shifting of the oral aperture.
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.
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, 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.
The orders included in the various classes are not discussed here, but are treated of under the following titles: - Peripatus (Onychophora), Centipede and Millipede (Myriapoda), Hexapoda (Insecta), Arachnida and Crustacea.
- Diagram to show the derivation of the unit or "ommatidium " of the compound eye of Crustacea and Hexapoda, C, from a simple monomeniscous monostichous eye resembling the lateral eye of a scorpion, A, or the unit of the compound lateral eye of Limulus (see article Arachnida, figs.
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