Chapter X: Arachnida (continued)—delobranchiata = Merostomata—xiphosura

SUB-CLASS I.—DELOBRANCHIATA = MEROSTOMATA.

=Order I. Xiphosura.=[206]

In his recent classification of the Arachnida, Lankester[207] has grouped the Xiphosura or King-crabs with the extinct Eurypterids or Gigantostraca under the name of Delobranchiata, better known under the name Merostomata[208] 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;[209] 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.

FIG. 152.—Dorsal view of the King-crab, _Limulus polyphemus_, × ½.
From Shipley and MacBride. 1, Carapace covering prosoma; 2, meso-
and metasoma; 3, telson; 4, median eye; 5, lateral eye.
]

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.

FIG. 153.—Ventral view of the King-crab, _Limulus polyphemus_, × ½.
From Shipley and MacBride. 1, Carapace covering prosoma; 2, meso-
and metasoma; 3, telson; 4, chelicera; 5, pedipalp; 6, 7, 8, 9, 3rd
to 6th appendages, ambulatory limbs; 10, genital operculum turned
forward to show the genital apertures; 11, 12, 13, 14, 15,
appendages bearing gill-books; 16, anus; 17, mouth; 18, chilaria.
]

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 │Postoral │ „
│ Legs │ │
VI. „ │2nd Walking │ „ │ „
│ Legs │ │
VII. „ │3rd Walking │ „ │ „
│ Legs │ │
VIII. „ │4th Walking │ „ │ „
│ Legs │ │
IX. „ │Chilaria │ „ │ „

X. „ │Genital │ „ │Mesosoma
│ operculum │ │
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[210] 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[211] 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[212] 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.[213] 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,[214] 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,[215] 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.”

FIG. 154.—A sagittal section of _Limulus_, seen from the right side,
somewhat smaller than natural size. After Patten and Redenbaugh.

All the prosomatic appendages, except the chelicera (4) and chilarium
(33) of the right side, are omitted. The genital operculum (32) and
the five gills (28) are represented.

The muscles are omitted except the fibres running from the occipital
ring to the posterior side of the oesophagus, the chilarial muscles,
the sphincter ani (27), and the levator ani (24).

The endosternite (34), with the occipital ring and the capsuliginous
bar, is seen from the side, and the positions of the abdominal
endochondrites (31) are indicated.

The mouth (1) leads into the oesophagus, which passes through the
brain to the proventriculus (12). A constriction, which marks the
position of the pyloric valve, separates the proventriculus from the
intestine (23) which passes posteriorly to the anus (26). A pair of
hepatic ducts (15) enter the intestine opposite the endocranium.

The heart (16) surrounded by the pericardial sinus lies above the
intestine. The pericardium is shown between the heart and the
intestine. The ostia (17) of the heart and the origins of the four
lateral arteries (19) are indicated; the frontal artery (13) and the
aortic arches (14) curving down to the brain, arise from the
anterior end of the heart; the superior abdominal artery and the
opening of the collateral artery into it are shown.

The brain surrounding the oesophagus is seen in side view upon the
neural side of the endosternite (34). The ventral cord (35) passes
through the occipital ring into the abdominal region. The anterior
commissure (3), with the three rostral nerves (2) innervating the
rostrum, or labrum, and four of the post-oral commissures, are
represented.

The cheliceral nerve with the small external pedal branch is shown
entire, but the next five neural nerves are cut off. The chilarial
nerve, the opercular nerve, and the five branchial nerves, enter
their respective appendages, the two former passing through the
occipital ring.

From the fore-brain the three olfactory nerves (5) pass anteriorly to
the olfactory organ; the median eye-nerve (10) passes to the right
of the proventriculus (12) to the median eyes (11); the lateral
eye-nerve (7) passes forward and is represented as cut off opposite
the proventriculus. The lateral nerve (9) or first haemal nerve is
also cut off just beyond the point where it fuses with the second
haemal nerve (8). The stomodaeal nerve (6) ramifies over the
oesophagus and proventriculus.

The second haemal nerve (8) passes to the anterior extremity of the
carapace; its haemal branch is cut off opposite the proventriculus.
An intestinal branch arises from near its base and disappears behind
the anterior cornu of the endosternite.

The next three haemal nerves (36) are cut off close to the brain, and
the following nine haemal nerves are cut off beyond the cardiac
branches. The fifteenth haemal nerve (29) is cut off beyond its
branch to the telson muscles. Both branches of the haemal nerve are
represented extending into the telson (25).

The intestinal nerves are shown arising from the haemal nerves and
entering the intestine. Those from the sixth and seventh neuromeres
pass through foramina in the endosternite, and communicate with a
plexus in the longitudinal abdominal muscles before entering the
intestine. The eighth passes just posterior to the endosternite and
joins the same plexus. Those from the first four branchial
neuromeres arise very near the abdominal ganglia, and are double in
their origins, the anterior branches joining the above-mentioned
plexus, and the posterior branches entering the intestine. The
fifteenth extends far back towards the rectum and anastomoses with
the sixteenth, which arises from the caudal branch of the sixteenth
haemal nerve, and innervates the rectum and anal muscles.

The segmental cardiac nerves (18) arise from the haemal nerves of the
sixth to the thirteenth neuromeres respectively. The most anterior
one passes to the inter-tergal muscles and the epidermis in the
median line, but the connections with the cardiac plexus have not
been made out. The next two (18) fuse to form a large nerve, which
passes to the inter-tergal muscles and epidermis, but has not been
observed to connect directly with the cardiac plexus. It, however,
sends posteriorly a branch, the pericardial nerve (20), which in
turn gives a branch to each of the cardiac nerves of the branchial
neuromeres, and then continues onward to the posterior margin of the
abdomen. This nerve lies in the epidermis. The median and lateral
cardiac nerves (22 and 21) are seen upon the walls of the heart. The
five cardiac nerves from the branchial neuromeres pass, in the
epidermis, to the median line, and dip down to the median nerve (22)
of the heart opposite the last five pairs of ostia (17). They
communicate with the pericardial nerve (20) and also with the
lateral sympathetic nerve (30).

Two post-cardiac nerves pass from the first and second post-branchial
nerves to the epidermis posterior to the heart.

The last cardiac nerve and the two post-cardiac nerves give off
branches which anastomose with each other and innervate the
extensors of the telson.

The lateral sympathetic nerve (30) receives branches from all the
neuromeres from the eighth to the fourteenth, either through the
cardiac nerves or the haemal nerves, and innervates the
branchio-thoracic muscles, extending with these far into the
cephalothorax.

1, Mouth; 2, rostral nerve in labrum; 3, anterior commissure; 4,
chelicera; 5, olfactory nerves; 6, stomodaeal nerve; 7, lateral
eye-nerve; 8, 2nd haemal nerve; 9, lateral nerve; 10, median
eye-nerve; 11, median eye; 12, proventriculus; 13, frontal artery;
14, aortic arch; 15, anterior hepatic duct of liver; 16, heart; 17,
2nd ostium; 18, 7th and 8th segmental cardiac nerves; 19, one of the
lateral arteries; 20, pericardial nerve; 21, lateral cardiac nerve;
22, median cardiac nerve; 23, intestine; 24, levator ani muscle; 25,
telson; 26, anus; 27, sphincter ani muscle; 28, last branchial
appendage; 29, 15th haemal nerve; 30, lateral sympathetic nerve; 31,
8th abdominal endochondrite; 32, genital operculum; 33, chilarium;
34, endosternite; 35, ventral nerve-cord; 36, 6th haemal nerve; 37,
origin of 6th neural nerve.
]

_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[216] 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. 154), 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.

FIG. 155.—Diagram of the first gill of _Limulus_, from the posterior
side, showing the distribution of the gill-nerve to the gill-book
(about natural size). After Patten and Redenbaugh. 1, Inner lobe of
the appendage; 2, outer lobe of appendage; 3, median lobe of
appendage; 4, gill-book; 5, neural nerve of the ninth neuromere; 6,
internal branchial nerve; 7, gill-nerve; 8, median branchial nerve;
9, external branchial nerve.
]

_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[217] 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.”[218]

FIG. 156.—A view of the nervous system of _Limulus_ from below. (About
natural size.) After Patten and Redenbaugh.

The carapace is represented as transparent. The appendages have been
removed, but the outlines of the left entocoxites (6) have been
sketched in. The positions of the abdominal appendages are indicated
by the external branchial muscles (17), the branchial cartilages
(19), the tendinous stigmata (18), and the abdominal endochondrites
(21). In the cephalothorax (1) all the tergo-coxal and plastro-coxal
muscles have been dissected away, leaving the endosternite (11) with
the occipital ring exposed. One of the left tergo-proplastral
muscles (4) and the left branchio-thoracic muscles (16) are
represented. The longitudinal abdominal muscles are also seen. All
the muscles of the right side have been omitted except the
haemo-neural muscles (23), of which the last two are represented
upon the left side also. At the base of the telson the flexors (29)
and extensors (27) of the caudal spine are represented as cut off
near their insertions. The sphincter ani (26), levator ani, and
occludor ani (25), and their relations to the anus (28), are shown.

The oesophagus runs forward to the proventriculus (3). From this the
intestine (20) passes posteriorly.

The brain lies upon the neural side of the endosternite, and the
ventral cord (22) passes back through the occipital ring. The neural
nerves are cut off, but the left haemal nerves and those from the
fore-brain (12) are represented entire.

The first pair of neural nerves go to the chelicerae. The second to
sixth pairs go to the next five cephalothoracic appendages, which
are represented by the entocoxites (6). The seventh pair of neural
nerves go to the chilaria, and the eighth pair to the operculum. The
neural nerves from the ninth to the thirteenth arise from the
abdominal ganglia and innervate the five pairs of gills.

From the fore-brain a median olfactory nerve (9) and two lateral ones
(8) pass forward to the olfactory organ; a median eye-nerve (2)
passes anteriorly and haemally upon the right of the proventriculus
(3) to the median eyes; and a pair of lateral eye-nerves pass to the
lateral eyes (15).

The first haemal nerve, or lateral nerve, follows the general course
of the lateral eye-nerve, but continues posteriorly far back on to
the neural side of the abdomen.

The haemal nerves of the hind-brain radiate from the brain to the
margins of the carapace, and each one passes anterior to the
appendage of its own metamere. The integumentary portions divide
into haemal and neural branches, of which the haemal branches (5)
are cut off. Each haemal branch gives off a small nerve which turns
back toward the median line upon the haemal side of the body.

The haemal nerves of the accessory brain pass through the occipital
ring to the sides of the body between the operculum and the sixth
cephalothoracic appendage. The seventh innervates the posterior
angles of the cephalothorax, the eighth the opercular portion of the
abdomen. The next five haemal nerves arise from the five branchial
neuromeres, pass out anterior to the gills to the sides of the
abdominal carapace, and innervate the first five spines upon the
sides of the abdomen.

The first post-branchial nerve innervates the last abdominal spine;
the second post-branchial nerve and one branch of the third
post-branchial innervate the posterior angles of the abdomen and the
muscles of the telson; and the caudal branch of the third
post-branchial nerve innervates the telson.

Intestinal branches arise from all the haemal nerves from the sixth to
the sixteenth, and pass to the longitudinal abdominal muscles and to
the intestine.

Cardiac nerves arise from all the haemal nerves from the sixth to the
thirteenth. Six of the cardiac nerves communicate with the lateral
sympathetic nerve (24), which innervates the branchio-thoracic
muscles (16).

Two post-cardiac nerves arise from the first two post-branchial
nerves, and passing to the haemal side anastomose with a branch from
the last cardiac nerve, and innervate the extensors (27) of the
telson and the epidermis behind the heart.

1, Cephalothorax; 2, median eye-nerve; 3, proventriculus; 4,
tergo-proplastral muscles; 5, haemal branch of integumentary nerve;
6, entocoxites; 7, 2nd haemal nerve; 8, right olfactory nerve; 9,
median olfactory nerve; 10, intestine; 11, endosternite; 12,
fore-brain; 13, origin of 4th neural nerve; 14, lateral nerve; 15,
lateral eye; 16, branchio-thoracic muscles; 17, external branchial
muscles; 18, tendinous stigmata; 19, branchial cartilages; 20,
intestine; 21, abdominal endochondrites; 22, ventral cord; 23,
haemo-neural muscles; 24, lateral sympathetic nerve; 25, occludor
ani; 26, sphincter ani; 27, extensors of telson; 28, anus; 29,
flexors of telson; 30, lateral projections of abdomen; 31, nerves of
spines; 32, external branchial muscles.
]

FIG. 157.—The markings on the sand made by the female _Limulus_ when
depositing eggs. Towards the lower end the round “nests” cease to be
apparent, the king-crab being apparently exhausted. (From
Kishinouye.) About natural size.
]

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.”[219] A line of nests (Fig. 157) 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. 158), 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.

FIG. 158.—Dorsal and ventral view of the last larval stage (the
so-called Trilobite stage) of _Limulus polyphemus_ before the
appearance of the telson. 1, Liver; 2, median eye; 3, lateral eye;
4, last walking leg; 5, chilaria. (From Kingsley and Takano.)
]

_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.”[220] 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.[221] 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,[222] 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.”[223]

=Fossil Xiphosura.=[224]

FIG. 159.—=A.=, _Hemiaspis limuloides_, Woodw., Upper Silurian,
Leintwardine, Shropshire. Natural size. (After Woodward.) =B.=,
_Prestwichia_ (_Euroöps_) _danae_ (Meek), Carboniferous, Illinois, ×
⅔. (After Packard.)
]

_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. 159, 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. 160) 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. 159, B).

FIG. 160.—_Belinurus reginae_, Baily, Coal Measures, Queen’s Co.,
Ireland, × 1. (After Woodward).
]

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[225] 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,[226] 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.

EURYPTERIDA

BY

HENRY WOODS, M.A.

St. John’s College, Cambridge, University Lecturer in Palaeozoology.