54-6 CRUSTACEA. 



but has both the axis and lateral lobes of the pygidium distinctly 

 segmented. 



Division C. Merostomata. 



The members of this group are Crustaceans, often of gigantic 

 size, in which the mouth is furnished with mandibles and maxillae, 

 the terminations of which become walking or swimming feet, 

 and organs of prehension. Two orders are included under the 

 Merostomata — viz., the recent King-crabs {Xiphosura) and the ex- 

 tinct Eurypterids. By Professor Claus these orders are regarded 

 as forming a special division of Arthropods, with relationships to 

 the Arachnida, to which he applies the name of Gigantostraca. 

 Professor Ray Lankester has also shown that there are various 

 remarkable features of relationship between Limulus and the Scor- 

 pions ; and the same point has been brought out by Van Beneden 

 from his researches into the development of the King-crabs. More- 

 over, the investigations of Mr Benjamin Peach would show that 

 there are close and important relationships between the Scorpions 

 and the Palaeozoic group of the Eurypterids. 



Order I. Xiphosura (Pozcilopoda). — This order includes 

 Crustacea hi which the anterior segments are amalgamated to form a 

 large cephalic buckler ; upon the dorsal surface of which are placed the 

 eyes (when present). On the under side of the head-shield is placed 

 the mouth, which is furnished with a rudimentary metastoma, 

 and is surrounded by six pairs of appendages, the bases of all but 

 the first pair being modified to act as masticating organs. The seg- 

 ments behind the head-shield may be fused with one another, or may 

 be more or less completely free ; and they carry upon their under sur- 

 face a series of lamellar appe?idages which are branchial in function. 

 The last segment of the body forms a sivord-like movable telso?i. 



The only existing genus contained in the order Xiphosura is 

 Limulus, comprising the King-crabs or Horse-shoe Crabs. In this 

 genus (fig. 410) the anterior segments — usually regarded as repre- 

 senting the head and thorax — are fused to form a horse-shoe-shaped 

 cephalothoracic shield. The upper surface of this carries subcen- 

 trally a pair of large compound eyes, and also a pair of small larval 

 eyes placed in the middle line in front. On the under side of the 

 head-shield is the mouth, surrounded by six pairs of jointed limbs, 

 the last five pairs having their basal joints spinose, and thus 

 adapted to act as jaws. The first pair of appendages — often 

 regarded as representing the antennae — are placed in front of 

 the mouth and terminate in nipping-claws. The next four pairs of 

 appendages are also generally chelate, while each of the last pair 

 of legs is terminated by two flat spines, with a whorl of similar 

 spines proximally, being thus adapted for locomotion. Behind 



