//. ACERATA: LINGUATULIDA, TARDIGRADA. 



455 



ring; the sexual organs are very complicated, the males having 

 the openings in front, the females at the hinder end. 



The presence of these parasites in animals causes a profuse catarrh, 

 and the eggs pass out with the mucus. Falling on vegetation, these are 



FIG. 474. FIG. 475. 



FIG. 474. Larva of Pentastomum proboscideum. (After Stiles.) rf, stomach; c, gland 

 cells ; m, mouth ; st, stylet ; ?/, posterior larval hooks ; J, 2, legs. 



FIG. 475. Macrobiotus hufelandi, water bear. (After drawings by Greef and Plate.) 

 I-IV^ legs ; d, accessory glands ; m, stomach ; mfc, mouth capsule ; ov, ovary ; sp, 

 salivary glands ; st, stylets ; vm, excretory tubules ; blood cells in the body. 



liable to be eaten by various animals. The larvae (tig. 474) have a boring 

 apparatus in front and two pairs of legs, the latter being lost in the 

 metamorphosis except for the hooks. It is by no means certain that 

 these are degenerate arachnids. The points in favor of such a position 

 are about equally balanced by those against. Pentastomum. 



Usually associated with the Arachnida are two other groups of very 

 doubtful position, which until more definite knowledge is obtained, may 

 remain near them. 



Tardigrada. 



These are minute fresh-water forms, known to microscopists as 

 4 water bears ' (fig. 475), which owe their name to their slow motions. 

 They have four pairs of short, hooked legs, their sole Arachnidan charac- 

 ter. The genital ducts empty into the rectum ; the nervous system has 

 four ventral ganglia ; heart and respiratory organs are lacking. In de- 

 velopment they are remarkable for the large ccelomic pouches. In the 



