856 



CLASS V. ARACHNIDA. 



or in ditches. They are very widely distributed and very common 

 yet they are seldom seen. Their small size aids them to elude 

 observation, they average but 0'3 mm. to 1 mm. in length. 



The body of a Tardigrade is enclosed in a cuticle and this 

 and the underlying tissues are often transparent, and, as many 

 of the cells are large and symmetrically placed and above all 

 there is no connective tissue 'to blur the organs, they form a very 

 favourite object for the inspection of microscopists. The cuticle 

 is usually of a uniform thickness all over the body, but in Echi- 

 niscus it is thickened into certain dorsal plates. The forked 

 claws at the end of each of the eight legs are also cuticular. From 

 time to time the cuticle is cast and it is not unusual to find a 

 Tardigrade imprisoned in two cuticles the outer one of which 



he has cast but not yet thrown 

 off. Macrobiotus lays its egg 

 in its cast cuticle. 



The shape of a Tardigrade 

 varies within limits but it is 

 usually somewhat squat ; many 

 have roughly speaking about 

 the proportions of a land tor- 

 toise and their movements are 

 as deliberate as those of the 

 Chelonians. Ante r i o r 1 y is 

 situated the mouth and pos- 

 teriorly the anus. On each 

 side are four legs, the anterior 

 pair pointing forward, the two 

 middle pairs forward or out- 

 ward, the hindermost pair 

 backward, and each leg ter- 

 minates in two forked claws 

 which assist the creature to 

 crawl about (Fig 568). Lydella 

 is described as having two 

 joints to each leg, but with this 

 exception the legs are un- 

 jointed. 



The epidermal cells which underlie the cuticle are arranged 

 in very definite rows ; they form but a single layer and the con- 



FIG. 568. Macrobiotus schtdtzei (after 

 Greeff). O mouth ; Vm pharynx ; Md 

 stomach ; Spd salivary glands ; Ov ovary : 

 T Malpia;hian tubules ; Vs accessory 

 gland. 



