ARACHNOWEA. 293 



(*) With trachese. Harvest-mites {Trombidium), minute para- 

 sites often troublesome in summer. Water-mites (Atax ; 

 hydrachna). Atax ypsilophorus occurs on the gills of the 

 fresh-water mussel. Beetle-mites [Gamasus), often found 

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



Order 8. Linguatulida. Pentastomuiii tanioides. 



This strange animal is }3arasitic 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 moveable hooks near the 

 mouth. There are no sense-organs nor trachese. 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 is not so unlike an Arachnoid as the adult is. 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. 



Order 9. Tardigrada. 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 re- 

 sembling 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. They are the only hermaphrodite 

 Arachnoids, if they are Arachnoids. The eggs, which are enveloped in 

 the cast skin of the parent, undergo total segmentation, but little is 

 known in regard to their development. 



The water-bears are said to have great powers of successfully resisting 

 desiccation, but perhaps it is the enclosed eggs which do so, developing 

 rapidly when favourable conditions return. 



Some authorities dignify (8) and (9) as classes ; for reasons of practical 

 expediency I continue to call them orders. 



Branchiate Arachnoidea (Poecilopoda). 



Order 10. Xiphosura. King-Crab or Horse-shoe Crab 

 (Limulus). 



The King-crab or Horse-shoe crab lives on the muddy or 

 sandy shore of the sheltered bays and estuaries of N. America, 



