The cuticle and epidermis enclose a space in which the various internal organs lie. This space is traversed by numerous symmetrically disposed muscle-fibres, and contains a clear fluid—the blood—which everywhere bathes these organs. This fluid evaporates when desiccation takes place, and is soon replaced after rain; it forms no coagulum when reagents are added to it, and it probably differs but little from water. Floating in it are numerous corpuscles, whose number increases with age. In well-fed Tardigrades the corpuscles are packed with food-reserves, often of the same colour—green or brown—as the contents of the stomach, which soon disappear when the little creatures are starved.
The alimentary canal begins with an oral cavity, which is in many species surrounded by chitinous rings. The number of these rings and their general arrangement are of systematic importance. The oral cavity opens behind into a fine tube lined with chitin, very characteristic of the Tardigrada, which has been termed the mouth-tube. By its side, converging anteriorly, lie the two chitinous teeth, which may open ventrally into the mouth-tube, as in Macrobiotus hufelandi and Doyeria simplex, or may open directly into the oral cavity, as in Echiniscus, Milnesium, and some species of Macrobiotus. In some of the last named the tips of the teeth are hardened by a calcareous deposit. The hinder end of each stylet or tooth is supported by a second chitinous tooth-bearer,[[375]] and the movement of each is controlled by three muscles, one of which, running forwards to the mouth, helps to protrude the tooth, whilst the other two running upwards and downwards to the sheath of the pharynx, direct in what plane the tooth shall be moved.
The mouth-tube passes suddenly into the muscular sucking pharynx, which is pierced by a continuation of its chitinous tube. Roughly speaking, the pharynx is spherical; the great thickness of its walls is due to radially arranged muscles which run from the chitinous tube to a surrounding membrane. When the muscles contract, the lumen of the tube is enlarged, and food, for the most part liquid, is sucked in. Two large glands, composed of cells with conspicuous nuclei, but with ill-defined cell outlines, pour their contents into the mouth in close proximity to the exit of the teeth. The secretion of the glands—often termed salivary glands—is said in many cases to be poisonous.
The pharynx may be followed by a distinct oesophagus, or it may pass almost immediately into the stomach, which consists of a layer of six-sided cells arranged in very definite rows. In fully-fed specimens these cells project into the lumen with a well-rounded contour. Posteriorly the stomach contracts and passes into the narrow rectum, which receives anteriorly the products of the excretory canals and the reproductive organs, and thus forms a cloaca. Its transversely placed orifice lies between the last pair of legs. The food of Tardigrades is mainly the sap of mosses and other humble plants, the cell-walls of which are pierced by the teeth of the little creatures.
The organs to which an excretory function has been attributed are a pair of lateral caeca, which vary much in size according as the possessor is well or ill nourished. They recall the Malpighian tubules of such Mites as Tyroglyphus. Nothing comparable in structure to nephridia or to coxal glands has been found.
The muscles show a beautiful symmetry. There are ventral, dorsal, and lateral bundles, and others that move the limbs and teeth, but the reader must be referred to the works of Basse, Doyère,[[376]] and Plate[[377]] for the details of their arrangement. The muscle-fibres are smooth.
Fig. [253].—Brain of Macrobiotus hufelandi, C. Sch., × about 350. (From Plate.) Seen from the side. ap, Lobe of brain bearing the eye; ce, supra-oesophageal ganglion; d, tooth; Ga, first ventral ganglion; ga’, sub-oesophageal ganglion; k, thickening of the epidermis round the mouth; oc, eye-spot; oe, oesophagus; op, nerve running from the ocular lobe of the brain to the first ventral ganglion; ph, pharynx.
The nervous system consists of a brain or supra-oesophageal ganglion, whose structure was first elucidated by Plate, and a ventral chain of four ganglia. Anteriorly the brain is rounded, and gives off a nerve to the skin; posteriorly each half divides into two lobes, an inner and an outer. The latter bears the eye-spot when this is present. Just below this eye a slender nerve passes straight to the first ventral ganglion. The brain is continued round the oral cavity as a thick nerve-ring, the ventral part of which forms the sub-oesophageal ganglion, united by two longitudinal commissures to the first ventral ganglion. Thus the brain has two channels of communication between it and the ventral nerve-cord on each side, one by means of the slender nerve above mentioned, and one through the sub-oesophageal ganglion. The ventral chain is composed of four ganglia connected together by widely divaricated commissures. Each ganglion gives off three pairs of nerves, two to the ventral musculature, and one to the dorsal. The terminations of these nerves in the muscles are very clearly seen in these transparent little creatures, though there is still much dispute as to their exact nature.
The older writers considered the Tardigrada as hermaphrodites, but Plate and others have conclusively shown that they are bisexual, at any rate in the genus Macrobiotus. The males are, however, much rarer than the females. The reproductive organs of both sexes are alike. Both ovary and testis are unpaired structures opening into the intestine, and each is provided with a dorsal accessory gland placed near its orifice. In the ovary many of the eggs are not destined to be fertilised, but serve as nourishment for the more successful ova which survive.