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[[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.