No special circulatory or respiratory organs exist, and, as in many other simple organisms, there is no connective tissue.
Fig. [254].—Male reproductive organs of Macrobiotus hufelandi, C. Sch., × about 350. (From Plate.) a.ep, Epidermal thickening round anus; cl, cloaca; gl.d, accessory gland; gl.l, Malpighian gland; st, stomach; te, testis; x, mother-cells of spermatozoa.
The segmentation of the egg in M. macronyx is total and equal, according to the observations of von Erlanger.[[378]] A blastula, followed by a gastrula, is formed. The blastopore closes, but later the anus appears at the same spot. There are four pairs of mesodermic diverticula which give rise to the coelom and the chief muscles. The reproductive organs arise as an unpaired diverticulum of the alimentary canal, which also gives origin to the Malpighian tubules. The development is thus very primitive and simple, and affords no evidence of degeneration.
With regard to their position in the animal kingdom, writers on the Tardigrada are by no means agreed. O. F. Müller placed them with the Mites; Schultze and Ehrenberg near the Crustacea; Dujardin and Doyère with the Rotifers near the Annelids; and von Graff with the Myzostomidae and the Pentastomida. Plate regards them as the lowest of all air-breathing Arthropods, but he carefully guards himself against the view that they retain the structure of the original Tracheates from which later forms have been derived. He looks upon Tardigrades as a side twig of the great Tracheate branch, but a twig which arises nearer the base of the branch than any other existing forms. These animals seem certainly to belong to the Arthropod phylum, inasmuch as they are segmented, have feet ending in claws, Malpighian tubules, and an entire absence of cilia. The second and third of these features indicate a relationship with the Tracheate groups; on the other hand there is an absence of paired sensory appendages, and of mouth-parts. Von Erlanger has pointed out that the Malpighian tubules, arising as they do from the mid-gut, are not homologous with the Malpighian tubules of most Tracheates, and he is inclined to place this group at the base or near the base of the whole Arthropod phylum. They, however, show little resemblance to any of the more primitive Crustacea. The matter must remain to a large extent a matter of opinion, but there can be no doubt that the Tardigrades show more marked affinities to the Arthropods than to any other group of the animal kingdom.
Biology.—Spallanzani, who published in the year 1776 his Opuscules de physique animale et végétale, was the first satisfactorily to describe the phenomena of the desiccation of Tardigrades, though the subject of the desiccation of Rotifers, Nematodes, and Infusoria had attracted much notice, since Leeuwenhoek had first drawn attention to it at the very beginning of the century. In its natural state and in a damp atmosphere Tardigrades live and move and have their being like other animals, but if the surroundings dry up, or if one be isolated on a microscopic slide and slowly allowed to dry, its movements cease, its body shrinks, its skin becomes wrinkled, and at length it takes on the appearance of a much weathered grain of sand in which no parts are distinguishable. In this state, in which it may remain for years, its only vital action must be respiration, and this must be reduced to a minimum. When water is added it slowly revives, the body swells, fills out, the legs project, and gradually it assumes its former plump appearance. For a time it remains still, and is then in a very favourable condition for observation, but soon it begins to move and resumes its ordinary life which has been so curiously interrupted.
All Tardigrades have not this peculiar power of revivification—anabiosis, Preyer calls it—it is confined to those species which live amongst moss, and the process of desiccation must be slow and, according to Lance,[[379]] the animal must be protected as much as possible from direct contact with the air.
According to Plate, the Tardigrada are free from parasitic Metazoa, which indeed could hardly find room in their minute bodies. They are, however, freely attacked by Bacteria and other lowly vegetable organisms, and these seem to flourish in the blood without apparently producing any deleterious effects on the host. Plate also records the occurrence of certain enigmatical spherical bodies which were found in the blood or more usually in the cells of the stomach. These bodies generally appeared when the Tardigrades were kept in the same unchanged water for some weeks. Nothing certain is known as to their nature or origin.
Systematic.—A good deal of work has recently been done by Mr. James Murray on the Polar Tardigrades and on the Tardigrades of Scotland, many of which have been collected by the staff of the Lake Survey.[[380]] Over forty species have been described from North Britain.
The following table of Classification is based on that drawn up by Plate:—