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.)