ARACHNIDA. The Arachnida are a class of animals be longing to the phylum Arthropoda and comprising the scorpions, spiders, mites and their allies, which are typically terrestrial air breeders, and also a host of marine forms, mostly extinct, of which the king crabs are the only existing representatives. The name was originally restricted to the terrestrial species resembling the in sects, centipedes, etc., in the possession of organs adapted for atmospheric respiration; king crabs possessing gills or branchia being for that reason associated with the Crustacea. But in 1881 Ray Lankester proved conclusively that the king crab ("Linnulus") must be regarded as an arachnid on account of the fundamental resemblance between it and the scorpion in all essential structural characters. This conclusion, which is generally accepted, focused attention upon the Trilobita (q.v.), a great group of extinct marine arthropods, previously classified as crustaceans but exhibit ing many points of likeness to the king crabs. In 1902 Lankester gave cogent reasons for the belief that the Trilobites were primi tive Arachnida and placed them in that class; and this opinion was confirmed by the subsequent discovery of numerous fossils linking them with king crabs and their allies. So far As external char acters are concerned—and these are the only characters known in the extinct species—the arthropods here referred to as the Arach nida agreed in the following particulars: (1) The body is divided into two main regions or parts, the prosoma or cephalothorax, and the opisthosoma or abdomen; (a) the prosoma, carrying the mouth, is composed of five or six somites or segments and is marked off from the opisthosoma by the fusion of more or fewer, generally all, of the dorsal plates or terga to form a heavy shield or carapace which bears the eyes, when present. It is usually attached to the opisthosoma by a movable joint and, excepting the Trilobites, is further separated from it by the structure and function of its appendages. (3) Ex cept in one degenerate family of mites, each segment of the pro soma is provided with a pair of appendages or limbs of which one pair only is situated in front of the mouth (preoral). The next are behind the mouth, or postoral, and the first pair of these at least is concerned with the mastication of food or with feeding in some other way. The remaining three or four pairs are mainly or wholly locomotor in function. (4) The opisthosoma and its ap pendages are much more variable in structure and agreed through out the different orders in two characters only, the anus opens upon the last segment and the genital ducts on the first or second. It is only by the position and function of the anterior appendages of the prosoma that the Arachnida mainly differ from the Crus tacea. In the Crustacea there are two pairs of preoral appendages, the second pair, corresponding to the first postoral pair in the Arachnida, having taken up a preoral position and lost their mas ticatory function, the third pair, not the second pair as in the Arachnida, being the first pair of jaws. Possibly some representa tives of the Trilobites will be found to link the Crustacea with the Arachnida, showing that we must look to the Trilobites as the common ancestors of the two classes.
According to the theory of the evolution of the Arachnida and the interpretation of this morphology propounded by Ray Lan kester and adopted in this article, the better known terrestrial forms, the scorpions, spiders and mites traced their descent and their structural characters from marine forms of which the Trilo bites are the oldest known rep resentatives.
In the Trilobites the prosoma is composed of five segments, with its dorsal plates welded into a carapace provided with corn pound lateral eyes. The mouth, situated near the middle of its lower surface, is bordered in front by a plate acting as an "upper lip" or labrum and behind by a small plate, the "lower lip" or labium. The preoral append ages are long, many jointed feelers or antennae, and the basal segments, or coxae, of the four pairs of postoral appendages act as jaws. The rest of the limb consists of two branches : an en dopodite, used for crawling, and an exopodite carrying a number of branchial filaments. The opis thosoma is very variable in the number of its segments ; but in the earliest forms there is a great number of them, all freely jointed together in a series, and each segment except the last car ries a pair of appendages approx imately like those of the prosoma in structure and function, the last or telson, carrying the anus, being sometimes provided with a post anal caudal spine.
The next stage was the restric tion of the number of segments of the body to 18 or 19, the loss, in most cases at all events, of appendages on some of the poste rior segments and a change in the structure in function of the ap pendages of the prosoma and opisthosoma, those of the pro soma losing to a great extent, if not wholly, their branchial func tion and becoming specialized for locomotion and the prehension and manipulation of food, those of the opisthosoma retaining for a time the double function of respiration and locomotion but later, as in the king crabs, losing their locomotive function and becoming specialized for the most part as carriers of the branchial plates at tached, to the branchial filaments of the Trilobites, to the outer branch or exopodite.
In the stage represented by the king crabs and many of their extinct allies, the prosoma consists of six segments with six pairs of locomotor or prehensile limbs and the appendages of the opis thosoma are reduced to six pairs corresponding to the first six segments of this region in the adult. The appendages of the first of these segments cover the genital apertures and form the geni tal operculum. It probably carried branchial plates in some forms, but these have disappeared in the living king crab. It must be noted, however, that this segment, the genital, is morphologically the second segment of the opisthosoma because in the course of its development the king crab shows in front of the genital somite a transient somite, which cannot certainly be traced in the adult. This pregenital somite is impor tant because it appears again in some of the terrestrial Arach nida but its occurrence is so in constant that the genital somite is regarded as the first of this region in this article. The limb bearing region of the opistho soma is called the mesosoma in contrast with the metasoma, the limbless region of the body behind it, which carries the anus and the caudal spine.
The most important allies of the king crab as witnesses of the kinship between that group and the terrestrial air-breathing Arachnida are the Eurypterida which resemble scorpions much more closely than the king crab resembles them, namely in shape, in identity in the number of somites, 18, exhibiting precise differentiation into the three categories prosoma, mesosoma and metasoma, composed of six somites each and the narrowing of the freely jointed, limbless metasoma to form, with the postanal spine, a tail-like termination to the body. But since only the exo skeleton of the Eurypterida is known, we must look to the king crabs for the evidence of relationship in the internal organs.
The following are the principal anatomical resemblances be tween them : (I) The alimentary canal is provided with a power ful suctorial pharynx, worked by special muscles, and with lateral segmentally-arranged diverticula, the so-called "gastric glands." (2) The vascular system is highly developed, consisting of a dor sal many-chambered "heart," from which arises a rich supply of arteries enveloping or intimately connected with the central nerv ous system. There is also a paired series of veno-pericardial muscles passing from the great ventral venous sinus to the peri cardium and constituting a mechanism aiding flow of blood along the veins which pass from the sinus to the heart. (3) On each side of the prosoma there is an excretory gland, the "coral gland," which opens by a pore between the basal segments (coxae) of the fifth and sixth ap pendages. (4) Lodged in the prosoma between the ventral nerve-mass and the alimentary canal is an internal skeletal plate, the "entosternite," a fibro-carti laginous structure giving support to the numerous muscles of the limbs and other organs and aris ing apparently from the solida tion of connective tissue and of the inner portion of the great dorso-ventral and longitudinal and crural muscles. (5) The ovaries and testes form a closed network and are not in the form of simple, or simply branched, tubes. (6) Median and lateral eyes are present. The medians are composed of two layers of cells (diplostichous) and the laterals of a single layer (monostichous), the elements of the lateral eyes in the scorpions being separated, whereas in the king crabs they are coalesced to form a compound eye.
The principal structural differences between the typical scor pions on the one hand and the king crabs and Eurypterida on the other are concerned with adaptations to life on the land, a new departure which marked a further stage in the evolution of the class. Of these adaptations atmospheric respiration is the ma important. As already explained the respiratory organs of the kir crabs are clusters of branchial lamellae, called gill-books, of whic there is a pair upon the second to the fifth appendages of the opi thosoma. In the scorpions the respiratory organs are four pail of small sacs, containing clusters of super-imposed lamellae, an they open by spiracles upon the sternal plates of the third to tt sixth somites of the opisthosoma. These lamellae, called luny books, are developed in the embryo behind the base of the foi, pairs of transient bud-like limbs. They closely resemble even i microscopic details the gill-books of the king crabs : and Lar kester's query that they are modified insunk branchial lamellae generally accepted. But in the embryo scorpion there is also pair of bud-like limbs in the first and second segments of the opi: thosoma. These persist, the first pair developing into a double c single plate, the genital operculum, and the second into a pair c peculiar tactile organs, called the combs or pectines, which cor sist of a jointed shaft provided with a series of teeth, the whol structure recalling and evidently representing the branchial exopc Bite of the Trilobite limb. Thus in their structure and develoc ment the pectines and lung-books of the scorpions bear out th view of the descent of this group from marine forms with branchia plates on the exopodite of five pairs of limbs of the opisthosoma and further evidence of this head is supplied by the discovery Silurian strata of a scorpion which lived in the sea and in a meas ure links the typical scorpions with the Eurypterida.
Less important adaptations to terrestrial life in scorpions ar certain modifications of the last four pairs of limbs of the prosom; for walking on land and the forward shifting of the mouth fron the middle of the lower side of the prosoma to the anterior end thus freeing the basal segments of the last two pairs of limbs fron function of manipulating or masticating food which they perforn in the king crabs and Eurypterida.
Although according to the theory of the evolution of the Arach nida here briefly sketched, the scorpions are the most primitiv( of all the terrestrial orders of the class, they cannot be regardec as the direct ancestors of any one of them. Nor can any of thesf other orders, the most specialized or the most degenerate, bein€ directly derived from another. In some cases they exhibit puz. zling gross resemblances associated with profound structural differ ences. Throughout the series a general tendency is trace able towards the shortening and simplification of the opisthosoma, usually called the abdomen, towards the disappearance or fusion of its segments, its coalescence with the prosoma or cephalo thorax, obliteration of the distinction between mesosoma and metasoma; and these changes are accompanied by simplification of the digestive, nervous and alimentary systems. The eyes also tend to disappear ; but apart from them the organs connected with the cephalothorax are generally at least as elaborate as in the scorpions, sometimes more so, and these call for special mention.
Excretory Organs.—Excretory tubules physiologically similar to those of insects but morphologically different from them open into the gut in the abdomen in scorpions, Pedipalpi, Araneae and Solifugae, but are absent in the Opiliones. The coxal glands have been found in all the orders; but they are more elaborate in the Pedipalpi and Araneae than in the Scorpiones. The position of the orifice varies. In some (Chelonethi, Mygalomorph spiders) it is close to the base of the fifth appendage as in scorpions and king crabs; in others (Pedipalpi, typical Araneae, Palpigradi, Solifugae) it is behind the base of the third appendage. No doubt in the ances tor of the Arachnida, possibly in Trilobites, there was a pair of coxal glands in each somite of the prosoma.
Entosternite.—The most highly developed entosternite is found in the Pedipalpi and Araneae, which also contain the prim itive pregenital somite. In these orders this plate is more like the entosternite of the king crabs than it is in the scorpions; in the Solifugae it is largely, if not wholly, replaced by a chitinous ingrowth (entosclerite) from the ventral surface.
The Generative Organs.—The essential glands, testes and ovaries are usually simple and tubular but in the Chelonethi they are joined by transverse bands and are reticulated, as in the scorpions and king crabs. Some remarkable phenomena connected with the copulation of the Arachnida may be referred to in this connection. In the king crabs fertilization is effected after the eggs are laid; but in the air-breathing forms the eggs are fertilized within the body of the female but the sperm of the male is intro duced in a variety of singular ways. The scorpions copulate front to front, the genital orifices of the two sexes being mutually ap plied during the process. The same method is probably adopted by the Pedipalpi and Chelonethi. In the Opiliones, so far as is known, the male and female stand facing one another and the male thrusts forward his penis, which is relatively of great length, be neath the cephalothorax of the female into her generative orifice which in many genera opens only a short distance behind the mouth. In other orders, however, the method of pairing is quite different, one of the pairs of cephalothoracic appendages being modified as an intromittent organ. In the spiders (Araneae), the terminal segment of the second pair, or palpi, is furnished with an apparatus adapted for taking up the liquid sperm after it has been deposited on a sheeting of web, and carrying it until the male finds a female and fertilizes her by inserting the organ into her generative ducts. Functionally similar organs occur upon the terminal segment or tarsus, of the fifth pair of limbs in the Rici nulei and upon the mandibles or chelicerae of the Solifugae. But in these cases the apparatus is adapted for the transmission of spermatophores into the female.
Spiracles are also present in the cephalothorax and are equally variable in position. In the Ricinulei there is a single pair of tufted tracheae opening above the base of the appendages of the fifth pair, and in the Solifugae a pair of tubular tracheae open ventrally behind the base of the appendages of the fourth pair. In the Acari, in which the tracheae may be tufted or tubular, spir acles may open above the appendages of the first pair, above the fourth pair or in the articular sockets of the third, fourth, fifth or sixth pairs; and in some Opiliones there is a spiracle from the fifth segment (tibia) of the last four pairs of appendages.
The Tracheae and the Ancestry of Air-Breathing Arach nida.—The tracheae are a morphological puzzle upon the solu tion of which two diametrically opposed hypotheses of the descent of the Arachnida mainly rest. In accordance with the view that the lung-books of the scorpions, Pedipalpi and some Araneae (spiders) were derived from branchial lamellae like those of the king crabs, their presence upon the first and second abdominal somites in the Pedipalpi and Araneae and upon the third to the sixth in the scorpions suggests that in earlier members of the king crab and Eurypterine stock they were also present upon the geni tal operculum, making altogether six pairs of clusters. It also in volves Lankester's conclusion that the lung-books were the primi tive type of respiratory organs in the air-breathing form, that they were subsequently partly suppressed or replaced by tracheal tubes on the abdomen and were functionally supplemented by addi tional tracheae with spiracle opening upon various parts of the cephalothorax; and the belief that these cephalothoracic spiracles and tracheae are adventitious organs gained support from their presence on the tibia of the legs of some Opiliones.
This view of the matter has not, however, been accepted by all authorities. Leuckart, and fol lowing him, Hansen and Soren sen, believed that segmentally arranged typical tubular tra cheae, like those of insects, were the earliest form of respiratory organs in the Arachnida and that from these they derived the tufted tracheae and from the tufted tracheae, lung-books. The primitive Arachnida in fact were terrestrial air-breathers. Hansen also believed that in these primi tive forms the cephalothorax was divided into two regions, a head, or cephalon, composed of four somites and bearing the eyes, mouth and four pairs of append ages, and a thorax consisting of two free somites with separate terga and two pairs of append ages. The acceptance of this opinion compels the belief that the Palpigradi are the most prim itive of existing Arachnida so far as the cephalothorax is concerned, with the Solifugae standing next to them. Bernard went a step farther and pictured the primi tive Arachnida as a terrestrial air-breathing arthropod composed of 18 somites, each provided with a pair of limbs, approximately alike throughout the series, a pair of tracheal spiracles and alimen tary diverticula.
These theories need not be further discussed. But it may be pointed out that neither involves a denial of the kinship between the scorpions and the king crabs and their allies because the branchial lamellae of the latter may have been derived from the pulmonary lamellae of the former. They involve, however, the conclusion that the king crabs and Eurypterida were derived from air-breathing land forms which became adapted to living in the sea, a conclusion diametrically opposed to the conclusion upheld in this article which maintains their descent from the Trilobites whose kinship with the king crabs was never questioned until the affinities of the latter with scorpions were established.
The Arachnida are classified as follows:— Grade 1. Anomomeristica.—Number of somites typically great and variable, usually exceeding eighteen. All the somites of the opisthosoma except the last with a pair of appendages struc turally and functionally like the postoral appendages of the pro soma. The dorsal area trilobite.