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PHYLUM ARTHROPODA 



There are no sense organs nor tracheae, nor is there any heart. The 

 sexes are separate ; the males smaller than the female. 



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. There is very 

 little reason for ranking them along with Arachnoids. 



Order Tardigrada. Water-bears or Sloth-animalcules, 



e.g. Macrobiotus 



Microscopic animals, sometimes foimd about the damp moss of 

 swamps or even in the roof-gutters of houses. Some occur in fresh 

 water, others in the sea. The unsegmented body is somewhat worm- 

 like, with four pairs of unjointed clawed limbs like little stumps, with 

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

 and sucking. The muscles are unstriped. 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 Tardigrada, even as adults, have great powers of 

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

 developing rapidly when favourable conditions return. There is very 

 little reason for ranking them along with Arachnoids. Perhaps, as the 

 seta-like " claws " and the cirri of some types suggest, they are nearer 

 to Annelids. 



Class Pal^ostraca 



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. The well-developed antennae of Trilobites, to- 

 gether with the markedly biramose character of some of 

 their limbs, suggest an affinity with Crustacea, but, on the 

 other hand, the affinities of the Xiphosura seem to be 

 distinctly Arachnoid. 



Order i. Xiphosura 



There is one living genus, the King-crab or Horseshoe- 

 crab {Limulus). 



The King-crab lives at slight depths off the muddy or 



