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.