PALJEOSTRACA, 331 



(/;) With tracheoe. Harvest-mites (Trombidiuni), whose minute 

 hexapod larva; are troublesome parasites in summer on 

 insects, many mammals, and man. The so-called " red- 

 spider " ( Tetrarhynchus telearius) spins webs, and lives 

 socially under leaves. Water-mites, e.g. Hydrachna on 

 water-beetles, and Atax on gills of fresh-water mussels. 

 Beetle-mites (Gamasus), often found on carrion beetles. 

 Ticks (Ixodes), on dogs, cattle, etc. 



Aberrant Orders or Classes. 

 Order 8. LlNGUATULIDA. Pentastomum tanioides. 



This strange animal is parasitic in the nasal and frontal cavities of 

 the dog and wolf. It is worm-like in form, externally ringed, without 

 any oral appendages, but with two pairs of movable hooks near the 

 mouth. There are no sense organs nor tracheae. The sexes are separate ; 

 the males smaller than the females. 



Embryos within egg-cases pass from the nostrils of the dog. If they 

 happen to be swallowed by a rabbit or a hare, or it may be some 

 other mammal, the embryos hatch in the gut and penetrate to liver or 

 lung. There they encyst, moult, and undergo metamorphosis. The 

 final larval form has two pairs of short legs, and has been compared 

 to a larval mite. Liberated from its encystment, it moves about within 

 its host, but will not become adult or sexual unless its host be eaten 

 by dog or wolf. There are a few other species occurring in Reptiles, 

 Apes, and even man, but their history is not adequately known, and 

 the systematic position is very uncertain. 



Order 9. Tardigarda. Water-Bears or Sloth-animalcules, 

 e.g. Macrobiotus. 



Microscopic animals, sometimes found about the damp moss of 

 swamps or even in the roof-gutters of houses. The body is somewhat 

 worm-like, with four pairs of clawed limbs like little stumps, with 

 mouth-parts resembling those of some mites, and adapted for piercing 

 and sucking. There is no abdomen. There is a food canal, a brain, 

 and a ventral chain of four ganglia, sometimes even a pair of simple 

 eyes, but no respiratory or vascular organs. The sexes are separate ; 

 the males rarer and smaller. 



The terrestrial Tardigarda, even as adults, have great powers of suc- 

 cessfully resisting desiccation, but sometimes only the eggs do so, 

 developing rapidly when favourable conditions return. 



Class PaljEostraca. 



The three following orders, Xiphosura, Eurypterina, and 

 Trilobita, may be united under this title. They live or lived 

 in water, and have or had gills in association with the limbs. 



