THE KING-CRAB. 375 
lung. There they éncyst, 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, 
eg. 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. Perhaps, as the 
seta-like ‘‘ claws” and the cirri of some types suggest, they are nearer 
to Annelids. ; ; 
Class PALHOSTRACA 
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 antennz of Trilobites, 
together 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 1. XIPHOSURA 
There is one living genus, the King-crab or Horseshoe- 
crab (Limulus). 
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 
