ANATOMY 481 



supported by a second chitinous tooth-bearer, 1 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 out- 

 lines, 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 con- 

 sists 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 Tyroylyphus. 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, 

 Doyere, 2 and Plate 3 for the details of their arrangement. The 

 muscle-fibres are smooth. 



1 A. Basse, Zeitschr. wiss. Zool. Ixxx. , 1906, p. 259. 

 2 Ann. Sci. nat. (2), xiv., 1840, p. 269, and xvii., 1842, p. 193. 

 3 Zool. Jahrb. Anat. iii., 1889. This paper contains a bibliography. 

 VOL. IV 2 I 



