PAL^OSTRACA. 337 



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 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 

 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. They are the only hermaphrodite 

 Arachnoids, if they are Arachnoids. The eggs, which are developed 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. 



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 recently discovered antennae of Trilobites, together 

 with the markedly biramose character of some of their 

 limbs, suggests 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 Horse-shoe 

 Crab (Limulus). 



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