ANATOMY 



479 



Fig. 251. — Echiniscus spinulosus, C. Sch., x about 

 200, seen from the side. (From Doyere.) 



for histological research. One way to produce the above-men- 

 tioned 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. 



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 Macro- 

 bioti 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 surfiice, 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 con- 

 tain 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 pig- 

 ment in the epidermis, which increases as the animal grows old, 

 obscures the internal structures. It is generally brown, black, 

 or red in colour. 



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 



