Fig. [249].—Dorsal view of Echiniscus testudo, C. Sch., × 200, showing the four segments 1, 2, 3, 4. (From Doyère.)
Fig. [250].—Cast-off cuticle of Macrobiotus tetradactylus, Gr., × about 150, containing four eggs in which the boring apparatus of the embryo can be distinguished. (From R. Greeff.)
Many Tardigrades are very transparent; their cells are large, and arranged in a beautifully symmetrical manner; and since those of them that live in moss, and at times undergo desiccation, are readily thrown into a perfectly motionless state, during which they may be examined at leisure, it is not surprising that these little creatures have been a favourite object for histological research. One way to produce the above-mentioned stillness is partly to asphyxiate the animals by placing them in water which has been boiled, and covering the surface of the water with a film of oil.
Fig. [251].—Echiniscus spinulosus, C. Sch., × about 200, seen from the side. (From Doyère.)
The whole body is enclosed in a thin transparent cuticle, which must be pierced by a needle if it be desired to stain the tissues of the interior. As a rule the cuticle is of the same thickness all over the body, but in the genus Echiniscus the cuticle of the dorsal surface is arranged in thickened plates, and these plates are finely granulated. From time to time the cuticle is cast, and this is a lengthy process, so that it is not unusual to find a Tardigrade ensheathed in two cuticles, the outer of which is being rubbed off. The Macrobioti lay their eggs in their cast cuticle (Fig. [250]). The end of each of the eight legs bears forked claws of cuticular origin. The legs are not jointed except in the genus Lydella, where two divisions are apparent.
Within the cuticle is the epidermis, a single layer of cells arranged in regular longitudinal and transverse rows along the upper and under surface, where the cells are as uniformly arranged and as rectangular as bricks. The cells on the sides of the body are polygonal, and not in such definite rows. The nuclei show the same diagrammatic symmetry as the cells which contain them, and lie in the same relative position in neighbouring cells. In a few places, such as the end of each limb and around the mouth and arms, the cells of the epidermis are heaped up and form a clump or ridge. In some genera a deposit of pigment in the epidermis, which increases as the animal grows old, obscures the internal structures. It is generally brown, black, or red in colour.
Fig. [252].—Macrobiotus schultzei, Gr., × 150. (Modified from Greeff.) a, The six inner papillae of the mouth; b, the chitin-lined oesophagus; c, calcareous spicule; d, muscle which moves the spicule; e, muscular pharynx with masticating plates; f, salivary glands; g, stomach; h, ovary; i, median dorsal accessory gland; k, diverticula of rectum.