358 PHYLUM ARTHROPODA. 



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 found 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. 



Class PAL/EOSTRACA. 



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 

 limbsj 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 (Limultts). 



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

 sandy shores of the sheltered bays and estuaries of North 

 America, from Maine to Florida, in the West Indies, and 

 also on the Molucca Islands, etc., in the far East. The 

 body consists of a vaulted cephalothorax shaped like a 



