//. AC'ERATA: LINOUATUUDA, TARBIGRADA. 



455 



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

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



The presence of tliese parasites in animals causes a profuse catarrli, 

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



Flo. 47+. 



Fig. 475. 



Fig. 474.— Larva of Feittiistomum proboscirlpum. (After Stiles.) f/, stcmach; e, gland 

 cells ; jji, mouth ; .s(, stylet ; y, posterior larval hooks ; 1, 3, legs. 



Fig. i'li. — Macrobiotus h-ufel<(ndi, water hear. (After drawings by Greet and Plate.) 

 /-/t^, legs ; d, accessory glands; m, stomach ; nik, mouth capsule ; ov, ovary ; sp, 

 salivary glands ; s(, stylets ; vrn, excretory tubules ; blood cells in the body.' 



liable to be eaten by various animals. The larvas (fig. 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 tliat 

 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 defiu'te knowledge is obtained, may 

 remain near them. 



Tardifjrada. 



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

 'water bears' (fig. 475), which owe tlicir name to tlieir 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 cuelomic pouches. In the 



