144 INVERTEBRATE ANIMALS. 
king-crabs (Lzzulus), and some large extinct forms nearly 
allied to them. They are all distinguished by the fact that the 
appendages which are placed round the mouth act by their 
bases as jaws, but have their extremities developed into swim- 
ming-paddles, walking-feet, or nipping-claws. 
The King-crabs (fig. 66) constitute a special group called 
Xiphosura (Gr. xiphos, a sword; and oura, tail),* from the fact 
that the end of the abdomen is furnished with a long sword- 
like spine (fig. 66, #4). The mouth is surrounded by six pairs of 
Fig. 66.—Xiphosura. Limulus poly- Fig. 67—Eurypterida. Prerygotus 
_ phemus, viewed from below. Anglicus, restored. (After H. 
Woodward.) 
appendages, the bases of which are spinous and act as jaws, 
whilst their free extremities are developed into nipping-claws or 
chelz. The whole of the upper surface of the body is protected 
by a kind of buckler, composed of an anterior semicircular 
shield, and a posterior somewhat hexagonal plate, the under 
surface of which carries branchial plates, whilst the sword-like 
telson is jointed to its hinder margin. The king-crabs attain a 
