J. MURRAY OX WATER-BEARS, OR TARDIGRADA. 59 



found, the low power usually suffices to enable us to determine 

 the genus and also some of the most distinct species. 



As a rule, however, a moderately high power will be necessary 

 to see some of the smaller structures by which species are dis- 

 tinguished, and very often the opacity of the animal will prevent 

 the high power being used to any purpose, unless the animal be 

 subjected to some degree of pressure. This cannot be done while 

 the animal is surrounded by debris. The cover-slip must be slid 

 off, the debris cleared off with a brush, and the cover replaced, 

 along with a sufficiently large drop of water to prevent the 

 crushing of the specimen. 



The requisite degree of pressure can then be applied by drawing 

 off some of the water with blotting-paper, and when the pressure 

 is just right the animal can be rolled about into any position by 

 touching the edge of the cover-slip. By these means we may see 

 all that is necessary to determine the species. 



More elaborate means, such as staining and sectioning, need 

 only be employed when we are going more deeply into the 

 minute structure. 



Form and Structure of Tardigrada. 



Water-bears are microscopic, elliptical or cylindrical, grub-like 

 animals, varying in size between 1 mm. and y§ mm - (-V an d 

 2-i^ in.). They are obscurely segmented, have a distinct head, 

 and four pairs of legs, the last pair terminating the body. 

 Each leg bears one or more curved claws, most commonly four 

 in number. 



The alimentary system consists of a mouth adapted for suction, 

 a pair of stilettiform or lancet-shaped teeth, suitable for piercing, 

 which enter the mouth or the throat ; a tubular gullet leading 

 to the pharynx, which is a muscular bulb acting as a pump ; an 

 oesophagus, leading to the stomach ; cloaca, and anus, which opens 

 vent rally, between the third and fourth pairs of legs. Between 

 the skin and the alimentary canal the body-cavity is filled with 

 the body -fluid or blood, in which float large nucleated cells, the 

 fat cells, and sometimes smaller granules. 



The central nervous system consists of the brain and four 

 ventral ganglia. There is generally a pair of dark pigment- 

 spots or eyes. 



