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.
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.
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.
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.
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.
ARACHNIDA, the zoological name given in 1815 by Lamarck (Gr.
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.
- 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.
The first segment of the mesosoma of Scorpio and Limulus thus remains the first segment, and can be identified as such throughout the Eu-arachnida, carrying as it always does the genital apertures.
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.
- 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.
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.
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.
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 ?
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.
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.
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.
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.
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, assistant in the Natural History departments of the British Museum, for valuable assistance in the preparation of this article and for the classification and definition of the groups of Eu-arachnida here given.
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.
Monomeniscous eyes with a double (really triple) cell-layer formed by invagination, as in the Eu-arachnida, are present.
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 Eu-arachnida are divided into two grades with reference to the condition of the respiratory organs as adapted to aquatic or terrestrial life.
All Arachnida, including Limulus, feed by suctorial action in essentially the same way as Scorpio.
- 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.