Introduction to Arachnida, and Xiphosura

BY

A. E. SHIPLEY, M.A., F.R.S.

Fellow of Christ’s College, Cambridge, and Reader in Zoology in the University

Chapter IX. 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. 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 (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.

Chapter X. Arachnida ( Continued )—Delobranchiata = Merostomata—Xiphosura

Sub-Class I.—Delobranchiata = Merostomata.

Order I. Xiphosura.

In his recent classification of the Arachnida, Lankester has grouped the Xiphosura or King-crabs with the extinct Eurypterids or Gigantostraca under the name of Delobranchiata, better known under the name Merostomata of Dana. The chief character of this group, and one which differentiates it from all the animals placed together by Lankester in the group Embolobranchiata, is that they have gills patent and exposed. The Xiphosura are, in fact, with the exception of a few marine Mites, the only Arachnids which now live in the sea as did their allies the Eurypterids in Palaeozoic times. With a few fresh-water exceptions, all other Arachnids have taken to life on land, and with a change from water-breathing to air-breathing came a change in the respiratory system, the gills becoming “lung-books,” or possibly tracheae, or disappearing altogether.

A few years ago Pocock re-classified the Xiphosura, and his classification will be found on pp. 276, 277. It will be noticed that in his classification the generic name Limulus has disappeared. I have, however, retained it in this article, partly because I regard the name as so well established that every one knows what it denotes, and partly because in a group which contains confessedly very few species, differing inter se comparatively slightly, it seems unnecessary to complicate matters with sub-families and new names.

Looked at from above a Limulus presents a horse-shoe-like outline, from the posterior end of which projects a long spine. It is often called in America the Horsefoot-crab, but its common or vulgar name is the King-crab. Across the middle of the body is a joint, and this joint separates the prosoma from the meso- and metasoma which are in King-crabs fused together. The prosoma comprises all the segments up to and including the segment which carries the chilaria; the mesosoma begins with the segment bearing the genital pores, and ends with the last segment which bears appendages; the metasoma comprises all the segments posterior to the last segment which carries appendages. The prosoma corresponds with the “cephalothorax” of some authors, and the meso-plus the metasoma are equivalent to their “abdomen.”

Dorsally, then, the prosoma is a vaulted structure with a smooth, horse-shoe-shaped anterior and lateral margin. Its posterior edge, the line where the meso-plus the metasoma are hinged, is a re-entrant bay with three sides. The meso- and metasoma are in the King-crabs fused together and form a hexagon. Three sides of this hexagonal double region form the hinge, two form the lateral margins and slope inwards; these bear six fused and six-jointed spines which have a segmental value. The sixth or posterior side is indented, and its concavity forms the area to which the large post-anal, unsegmented tail or spine is hinged.

The whole body is covered by a smooth chitinous sheath varying from sage-green to black in colour, and it is kept very clean, probably by some excretion which hinders various sessile animals attaching themselves to it as they do, for instance, on many Copepods. Burrowing animals like Limulus are usually free from these messmates. King-crabs have a self-respecting, well-groomed appearance. On the rounded dorsal surface the chitinous covering is produced into a certain number of spines arranged in a median and two lateral rows. The anterior median spine overhangs the median eyes, and the anterior lateral spine on each side overshadows the large lateral eyes.

The vaulted carapace is turned in on the under side, where there is a flat rim which widens anteriorly, and on the inner edge this rim borders a sunken area, into the concavity of which the numerous appendages project. Thus, although when viewed from above a Limulus looks as though it had a solid body shaped something like half a pear, when viewed from below, especially if the appendages be removed, it will be found that the body is thin and hollowed, and almost leaf-like, as if most of the edible part of the half-pear had been scooped out. Within the hollow thus formed the appendages lie, and here they move about, seldom or never protruding beyond the edge of the carapace,—in fact, all except the pedipalps and ambulatory legs are too short to project beyond this limit.

The body of a King-crab can be analysed into twenty-one segments, but these do not all persist to the adult stage. They are grouped together in higher aggregates, or “tagmata” as Lankester calls them, and most of the segments bear paired appendages.

The segments with their respective appendages and their grouping into tagmata are shown in the following scheme:—

Appendages.

I. Segment Median eyes Preoral Prosoma

II. „ Rostrum „

III. „ Chelicerae „

IV. „ Pedipalpi Lateral to mouth

V. „ 1st Walking Legs Postoral

VI. „ 2nd Walking Legs „

VII. „ 3rd Walking Legs „

VIII. „ 4th Walking Legs „

IX. „ Chilaria „

X. „ Genital operculum „ Mesosoma

XI. „ 1st Gill-books „

XII. „ 2nd Gill-books „

XIII. „ 3rd Gill-books „

XIV. „ 4th Gill-books „

XV. „ 5th Gill-books „

XVI. „ No appendages „ Metasoma

XVII. „ „ „

XVIII. „ „ „

XIX. „ „ „

XX. „ „ „

XXI. „ „ „

We have followed Carpenter in inserting the rostral segment. This corresponds with the segment that in Insects and Crustacea bears the antennae or first antennae respectively, the absence of these organs being one of the characteristic but negative features of all Arachnids. The evidence for the existence of this evanescent segment rests partly upon the observation of von Jaworowski on the vestigial feelers in an embryo Spider, Trochosa, and perhaps more securely on the fact that, according to Korschelt and Heider, there is a distinct neuromere for this segment, between the proto-cerebral neuromere which supplies the eyes and the trito-cerebral neuromere which supplies the chelicerae. According to Brauer the chelicerae of Scorpions are also supplied by the third neuromere.

The bases of the chelicerae do not limit the mouth, but between and behind them is a ridge or tubercle which has the same relationship to the mouth of Limulus that the labrum has in Insects and some Crustacea. Posteriorly the mouth is bounded by the “promesosternite,” a large median plate which lies between the bases of the ambulatory limbs. The pedipalps and all the ambulatory limbs have their bases directed towards the mouth, their gnathobases or sterno-coxal processes are cushion-like structures covered with spines—all pointing inwards—and with crushing teeth. They form a very efficient manducatory apparatus. The boundary of the mouth is finally completed by the chilaria.

Certain of the appendages which persist will be described with the functions they subserve, the eyes with the sense-organs, the genital operculum with the generative organs, the gill-books with the respiratory system, but the chelicerae, pedipalpi, and walking limbs, which have retained the functions of prehension and locomotion usual to limbs, merit a little attention. The chelicerae are short and composed of but three joints. They are, like the succeeding segments, chelate, and the chelae of all are fine and delicate like a pair of forceps rather than like a Lobster’s claw. In the female L. polyphemus the pedipalp is remarkably like the three ambulatory legs which succeed it, and all four are chelate, but in the adult male the penultimate joint of the pedipalp is not prolonged to form one limb of the chela, which is therefore absent, and the appendage is thicker and heavier than in the other sex. In L. longispina and L. moluccanus the first walking leg, as well as the pedipalp, ends in a claw and not in a chela; the immature males resemble the females. The first three walking legs in both sexes of L. polyphemus resemble the pedipalpi of the female, and like them have six joints. The fourth and last pair of ambulatory appendages is not chelate, but its distal joints carry a number of somewhat flattened structures, which are capable of being alternately divaricated and approximated or bunched together. This enables them to act as organs for clearing away sand or mud from beneath the carapace as the creature lies prone on the bottom of the sea. To quote Mr. Lloyd, the “two limbs are, sometimes alternately and sometimes simultaneously, thrust backward below the carapace, quite beyond the hinder edge of the shell; and in the act of thrusting, the lobes or plates on each leg encounter the sand, the resistance or pressure of which causes them to open and fill with sand, a load of which at every thrusting operation is pushed away from under the king-crab, and deposited outside the carapace. The four plates then close and are withdrawn closed, previous to being opened and charged with another load of sand; and at the deposit of every load the whole animal sinks deeper into its bed, till it is hidden all except the eyes.” There seems little doubt that the action of these appendages in removing the sand from under the carapace is reinforced by the fanning action of the respiratory appendages, which set up a current that helps to wash the particles away. But the posterior walking legs are not the only organs used in burrowing. The Rev. Dr. Lockwood, who observed the habits of L. polyphemus off the New Jersey coast, says, “The king-crab delights in moderately deep water, say from two to six fathoms. It is emphatically a burrowing animal, living literally in the mud, into which it scoops or gouges its way with great facility. In the burrowing operation the forward edge of the anterior shield is pressed downward and shoved forward, the two shields being inflected, and the sharp point of the tail presenting the fulcrum as it pierces the mud, whilst underneath the feet are incessantly active scratching up and pushing out the earth on both sides. There is a singular economy of force in this excavating action; for the doubling up or inflecting and straightening out of the two carapaces, with the pushing purchase exerted by the tail, accomplish both digging and subterranean progression.”

At night-time Limulus is apt to leave the sand and progress by a series of short swimming hops, the respiratory appendages giving the necessary impetus, whilst between each two short flights the animal balances itself for a moment on the tip of its tail. During this method of progressing the carapace is slanting, forming an angle of about 45° with the ground. The unsegmented tail is also used when a King-crab falls on its back. “The spine is then bent, i.e. its point is planted in the sand so that it makes an acute angle with the carapace, which is then so far raised that some of the feet are enabled to grasp a projecting surface, either longitudinal or vertical, or at some combination of the two; and the crab then turns over.”

Limulus feeds partly on bivalves, but mainly on worms, especially Nereids, which it catches with its chelate limbs as it burrows through the sand. The food is held immediately under the mouth by the chelicerae, aided at times by the succeeding appendages; it is thus brought within range of the gnathobases of the walking legs, and these by an alternate motion “card” the food into fragments, which when sufficiently comminuted pass into the mouth. At times its appendages are caught between the valves of Venus mercenaria, a burrowing bivalve known in America as the “quahog” or “round clam.” The Limulus has seized with its chelate claws the protruding siphon of this mollusc, which, being rapidly drawn in, drags with it the limb of the king-crab, and the valves of the clam are swiftly snapped to.

As a rule in Arachnids the alimentary canal is no longer than the body, and runs straight from mouth to anus, but in Limulus, the mouth being pushed far backward, there is a median loop, and the narrow oesophagus which leads from the mouth, having traversed the nerve-ring, passes forward towards the anterior end of the carapace. Here it enters into a somewhat ⸧ shaped and spacious proventriculus; posteriorly the proventriculus opens by a funnel-shaped valve into the anterior end of the narrow intestine. All these structures are derived from the stomodaeum, are lined with chitin and are provided with very muscular walls whose internal surface is thrown into longitudinal ridges. The intestine runs straight backward, diminishing in its diameter, and ends in a short, chitin-lined, and muscular rectum which is derived from the proctodaeum; the anus is a longitudinal slit. A large gland, usually called the liver, consisting of innumerable tubules, pours its secretions into the broader anterior end of the intestine by two ducts upon each side; it extends into the meso- and metasoma, and, together with the reproductive organs, forms a “packing” in which the other organs are embedded. The contents of the alimentary canal are described as “pulpy and scanty,” and probably much of the actual digestion goes on inside the lumen of the above-mentioned gland.

The vascular system of Limulus, like that of the Scorpions, is more completely developed than is usually the case in Arthropods. For the most part the blood runs in definite arteries, and when it passes as it does into venous lacunae these are more definite in position and in their retaining walls than in other members of the phylum.

The heart lies in a pericardial space with which it communicates by eight pairs of ostia. Eight paired bands of connective tissue, the “alary muscles” of authors, sling the heart to the pericardial membrane. Posteriorly the pericardial chamber receives five paired veins on each side coming from the gills and returning the purified blood to the heart.

Eleven arteries arise from the heart. These are (i.) a median frontal artery which, passing forward, divides into a right and left marginal artery. These run round the edge of the carapace to its posterior angle, where each receives a branch of the collateral artery mentioned below. (ii.) and (iii.) are the aortic arches (Fig. ), paired vessels running round and supplying the proventriculus and oesophagus. These unite ventrally in a vascular ring which encloses the nerve-ring, and is continued along the ventral nerve-cord as the ventral artery and along some of the chief nerves. This vascular ring supplies the lateral eyes and all the appendages mentioned on p. 263 up to and including the genital operculum. The ventral artery supplies the respiratory appendages, and gives branches to the rectum, caudal spine, etc. Two of its branches encircle the rectum, and uniting open into the superior abdominal artery. iv.–xi. are paired lateral arteries which leave the heart beneath the anterior four ostia, and soon enter a longitudinal pair of collateral arteries which unite behind in the just mentioned superior abdominal artery; they also give off branches to the muscles and to the intestine, and a stout branch mentioned above which passes into the marginal artery posteriorly. The venous system is lacunar, and the blood is collected from the irregular spaces between the various organs into a pair of longitudinal sinuses, whence it passes into the operculum and the five pairs of gills. A large branchio-cardiac canal returns the blood from each gill to the cavity of the pericardium, and so through the ostia to the heart. Eight veno-pericardiac muscles run from the under surface of the pericardium to be inserted into the upper surface of the longitudinal sinus; they occur opposite the ostia, and play an important part in the mechanism of the circulation. The blood is coloured blue by haemocyanin; amoeboid corpuscles float in the plasma.

The respiratory organs are external gills borne on the posterior face of the exopodite of the lamella-like posterior five mesosomatic limbs. Each gill consists of a series of leaves like the leaves of a book, and some 150–200 in number. Within the substance of each leaf the blood flows, while without the oxygen-carrying water circulates between the leaves. These gillbearing appendages can be flapped to and fro, and they seem to be at times held apart by the flabellum, a spatulate process which Patten and Redenbaugh regard as a development of the median sensory knob on the outer side of the coxopodite of the last pair of walking limbs.

Limulus has no trace of Malpighian tubules, structures which seem often to develop only when animals cease to live in water and come to live in air. The Xiphosura have retained as organs of nitrogenous excretion the more primitive nephridia, or coxal glands as they are called, in the Arachnida. They are redbrick in colour, and consist of a longitudinal portion on each side of the body, which gives off a lobe opposite the base of the pedipalps and each of the first three walking legs—in the embryo also of the chelicerae and last walking legs, but these latter disappear during development. A duct leads from the interior of the gland and opens upon the posterior face of the last pair of walking legs but one.

The nervous system has been very fully described by Patten and Redenbaugh, and its complex nature plays a large part in the ingenious speculations of Dr. Gaskell as to the origin of Vertebrates. It consists of a stout ring surrounding the oesophagus and a ventral nerve-cord, composed—if we omit the so-called fore-brain—of sixteen neuromeres. The fore-brain supplies the median and the lateral eyes, and gives off a median nerve which runs to an organ, described as olfactory by Patten, situated in front of the chelicerae on the ventral face of the carapace. Patten distinguishes behind the fore-brain a mid-brain, which consists solely of the cheliceral neuromere, a hind-brain which supplies the pedipalps and four pair of walking legs, and an accessory brain which supplies the chilaria and the genital operculum. This is continued backward into a ventral nerve-cord which bears five paired ganglia supplying the five pairs of gills and three pairs of post-branchial ganglia; the latter are ill-defined and closely fused together. As was mentioned above, the whole of the central nervous system is bathed in the blood of the ventral sinus.

The sense-organs consist of the olfactory organ of Patten, the median and lateral eyes, and possibly of certain gustatory hairs upon the gnathobases. The lateral eyes in their histology are not so differentiated as the median eyes, but both fall well within the limits of Arachnid eye-structure, and their minute anatomy has been advanced as one piece of evidence amongst many which tend to demonstrate that Limulus is an Arachnid.

Both ovaries and testes take the form of a tubular network which is almost inextricably entangled with the liver. From each side a duct collects the reproductive cells which are formed from cells lining the walls of the tubes, and discharges them by a pore one on each side of the hinder surface of the genital operculum. As is frequently the case in Arachnids the males are smaller than the females, and after their last ecdysis the pedipalps and first two pairs of walking legs, or some of these appendages, end in slightly bent claws and not in chelae. Off the New Jersey coast the king-crabs (L. polyphemus) spawn during the months of May, June, and July, Lockwood states at the periods of highest tides, but Kingsley was never “able to notice any connexion between the hours when they frequent the shore and the state of the tide.” “When first seen they come from the deeper water, the male, which is almost always the smaller, grasping the hinder half of the carapace of the female with the modified pincer of the second pair of feet. Thus fastened together the male rides to shallow water. The couples will stop at intervals and then move on. Usually a nest of eggs can be found at each of the stopping-places, and as each nest is usually buried from one to two inches beneath the surface of the sand, it appears probable that the female thrusts the genital plate into the sand, while at the same time the male discharges the milt into the water. I have not been able to watch the process more closely because the animals lie so close to the sand, and all the appendages are concealed beneath the carapace. If touched during the oviposition, they cease the operation and wander to another spot or separate and return to deep water. I have never seen the couples come entirely out of the water, although they frequently come so close to the shore that portions of the carapace are uncovered.”

The developing ova and young larvae are very hardy, and in a little sea-water, or still better packed in sea-weed, will survive long journeys. In this way they have been transported from the Atlantic to the Pacific coasts of the United States, and for a time at any rate flourished in the western waters. Three barrels full of them consigned from Woods Holl to Sir E. Ray Lankester arrived in England with a large proportion of larvae alive and apparently well.

According to Kishinouye, L. longispina spawns chiefly in August and between tide-marks. “The female excavates a hole about 15 cm. deep, and deposits eggs in it while the male fertilises them. The female afterwards buries them, and begins to excavate the next hole.” A line of nests (Fig. ) is thus established which is always at right angles to the shore-line. After a certain number of nests have been formed the female tires, and the heaped up sand is not so prominent. In each “nest” there are about a thousand eggs, placed first to the left side of the nest and then to the right, from which Kishinouye concludes that the left ovary deposits its ova first and then the right. Limulus rotundicauda and L. moluccanus do not bury their eggs, but carry them about attached to their swimmerets.

The egg is covered by a leathery egg-shell which bursts after a certain time, and leaves the larva surrounded only by the blastodermic cuticle; when ripe it emerges in the condition known as the “Trilobite larva” (Fig. ), so-called from a superficial and misleading resemblance to a Trilobite. They are active little larvae, burrowing in the sand like their parents, and swimming vigorously about by aid of their leaf-like posterior limbs. Sometimes they are taken in tow-nets. After the first moult the segments of the meso- and metasoma, which at first had been free, showing affinities with Prestwichia and Belinurus of Palaeozoic times, become more solidified, while the post-anal tail-spine—absent in the Trilobite larva—makes its first appearance. This increases in size with successive moults. We have already noted the late appearance of the external sexual characters, the chelate walking appendages only being replaced by hooks at the last moult.

Limulus casts its cuticle several times during the first year—Lockwood estimates five or six times between hatching out in June and the onset of the cold weather. The cuticle splits along a “thin narrow rim” which “runs round the under side of the anterior portion of the cephalic shield.” This extends until it reaches that level where the animal is widest. Through this slit the body of the king-crab emerges, coming out, not as that of a beetle anteriorly and dorsally, but anteriorly and ventrally, in such a way as to induce the unobservant to exclaim “it is spewing itself out of its mouth.” In one nearly full-sized animal the increase in the shorter diameter of the cephalic shield after a moult was from 8 inches to 9½ inches, which is an indication of very rapid growth. If after their first year they moult annually Lockwood estimates it would take them eight years to attain their full size.

The only economic use I know to which Limulus is put is that of feeding both poultry and pigs. The females are preferred on account of the eggs, of which half-a-pint may be crowded into the cephalic shield. The king-crab is opened by running a knife round the thin line mentioned on p. 275. There is a belief in New Jersey that this diet makes the poultry lay; undoubtedly it fattens both fowls and pigs, but it gives a “shocking” flavour to the flesh of both.

Classification.

But five species of existing King-crabs are known, and these are grouped by Pocock into two sub-families: (i.) the Xiphosurinae, and (ii.) the Tachypleinae. These together make up the single family Xiphosuridae which is co-extensive with the Order. The following is Pocock’s classification. The names used in this article are printed in italic capitals.

Order Xiphosura.

Family 1. Xiphosuridae.

Sub-Fam. 1. Xiphosurinae.

This includes the single species Xiphosura polyphemus (Linn.) (= Limulus polyphemus, Latreille), “which is said to range from the coast of Maine to Yucatan.”

Sub-Fam. 2. Tachypleinae.

Genus A. Tachypleus includes three species: (i.) T. gigas, Müll. (= Limulus gigas, Müll., and L. moluccanus, Latreille), widely distributed in Malaysia; (ii.) T. tridentatus, Leach (= L. tridentatus, Leach, and L. longispina, Van der Hoeven), extending from British North Borneo to China and Southern Japan; and (iii.) T. hoeveni, Pocock (= L. moluccanus, Van der Hoeven), found in the Moluccas.

Genus B. Carcinoscorpius with one species, C. rotundicauda (Latreille) (= L. rotundicauda, Latreille). It occupies a more westerly area than T. gigas or than T. tridentatus, having been recorded from India and Bengal, the Gulf of Siam, Penang, the Moluccas, and the Philippines.

With regard to the affinities of the group it is now almost universally accepted that they are Arachnids. The chief features in which they differ from other Arachnids are the presence of gills and the absence of Malpighian tubules, both being features associated with aquatic life. As long ago as 1829 Straus-Dürckheim emphasised the points of resemblance between the two groups, and although the view was during the middle of the last century by no means universally accepted, towards the end of that epoch the painstaking researches of Lankester and his pupils, who compared the King-crab and the Scorpion, segment with segment, organ with organ, tissue with tissue, almost cell with cell, established the connexion beyond doubt. Lankester would put the Trilobites in the same phylum, but in this we do not follow him. With regard to the brilliant but, to our mind, unconvincing speculations as to the connexion of some Limulus-like ancestor with the Vertebrates, we must refer the reader to the ingenious writings of Dr. Gaskell, recently summarised in his volume on “The Origin of Vertebrates,” and to those of Dr. Patten in his article “On the Origin of Vertebrates from Arachnids.”

Fossil Xiphosura.

Limulus is an example of a persistent type. It appears first in deposits of Triassic age, and is found again in the Jurassic, Cretaceous, and Oligocene. In the lithographic limestone of Solenhofen in Bavaria, which is of Upper Jurassic age, Limulus is common and is represented by several species. One species is known from the Chalk of Lebanon, and another occurs in the Oligocene of Saxony. No other genus of the Xiphosura appears to be represented in the Mesozoic and Tertiary deposits, but in the Palaeozoic formations (principally in the Upper Silurian, the Old Red Sandstone, and the Coal Measures) several genera have been found, most of which differ from Limulus in having some or all of the segments of the abdomen free; in this respect they resemble the Eurypterida, but differ from them in the number of segments. In Hemiaspis (Fig. , A), from the Silurian, the segments of the abdomen are divisible into two groups (mesosoma and metasoma) in the same way that they are in Eurypterids; the first six segments have broad, short terga, the lateral margins of the sixth being divided into two lobes, probably indicating the presence of two fused segments; the last three segments are narrower and longer than the preceding, and at the end is a pointed tail-spine. In Belinurus (Fig. ) from the Carboniferous, the two regions of the abdomen are much less distinct; there are eight segments, the last three of which are fused together, and a long tail-spine. In Neolimulus, from the Silurian, there seems to be no division of the abdomen into two regions, and apparently all the segments were free. On the other hand, in Prestwichia (Carboniferous), all the segments of the abdomen, of which there appear to be seven only, were fused together (Fig. , B).

In the Palaeozoic genera the median or axial part of the dorsal surface is raised and distinctly limited on each side, so presenting a trilobed appearance similar to that of Trilobites. In Neolimulus, Belinurus, and Prestwichia, lateral eyes are present on the sides of the axial parts of the carapace, and near its front margin median eyes have been found in the two last-named genera.

In nearly all the specimens of Palaeozoic Xiphosura which have been found nothing is seen but the dorsal surface of the body; in only a very few cases have any traces of the appendages been seen, but, so far as known, they appear to have the same general character as in Limulus.

Aglaspis, found in the Upper Cambrian of Wisconsin, has been regarded as a Xiphosuran. If that view of its position is correct, then Aglaspis will be the earliest representative of the group at present known. Other genera of Palaeozoic Xiphosura are Bunodes, Bunodella, and Pseudoniscus in the Silurian; Protolimulus in the Upper Devonian; and Prolimulus in the Permian.