ANNULOSA : CRUSTACEA. 187 



may cut the anterior margin of the shield separately in which case the 

 free cheeks will be discontinuous. The posterior angles of the free cheeks 

 are often produced into long spines. 



Behind the cephalic shield comes the thorax, composed of a variable 

 number of segments, which are not soldered together, but are capable of 

 free motion upon one another, so as to allow the animal to roll itself up 

 after the manner of a wood-louse, or hedgehog. The thorax is usually 

 strongly trilobed, and each thorax-ring shows the same trilobation, being 

 composed of a central, more or less strongly convex, portion, called the 

 ' axis,' and of two flatter side-lobes, called the ' pleurae.' 



The 'pygidium,' or 'tail,' is usually trilobed, also, and, like the thorax, 

 consists of a median axis and of a marginal limb, the composition of the 

 whole out of anchylosed segments being shown by the existence of axial 

 and pleural grooves. 



ORDER IV. MEROSTOMATA. The members of this order are 

 Crustacea, often of gigantic size, in which the mouth is fur- 

 nished with mandibles and maxillae, the terminations of which 

 become walking- or swimming-feet, and organs of prehension. 



This order comprises the recent King Crabs, and the extinct 

 Pterygoti and Eurypteri. 



SUB-ORDER I. XIPHOSURA (Pmcilopoda). 'Crustacea having 

 the anterior segments welded together to form a broad convex 

 buckler, upon the dorsal surface of which are placed the com- 

 pound eyes and ocelli ; the former sub- centrally, the latter in 

 the centre in front. The mouth is furnished with a small la- 

 brum, a rudimentary metastoma and six pairs of appendages. 

 Posterior segments of the body more or less free, and bearing 

 upon their ventral surfaces a series of broad lamellar appen- 

 dages; the telson, or terminal segment, ensiform.' (Henry 

 Woodward.) 



The Xiphosura include no other recent forms than the 

 Limuli (King Crabs, or Molucca Crabs) (fig. 53). They are 

 distinguished by the possession of six pairs of chelate limbs, 

 placed round the mouth, having their bases spinous and officiating 

 as jaws. Six other pairs of foliaceous appendages are attached 

 to the abdomen, and the last five of these carry branchia?. 

 The body, which is often of great size, when viewed from 

 above exhibits a division into three portions : (1) An ante- 

 rior semicircular shield, which carries two compound and two 

 simple eyes ; (2) a posterior, irregularly hexagonal shield 

 which covers the abdomen ; and (3) a long, sword- like telson, 

 articulated to the dorsal buckler, and giving the name to the 

 sub-order. 



The chief features, therefore, which characterise the Limulus 



are as follows : 1. The possession of six pairs of appendages 



which are placed round the mouth, have their bases spinous, 



act as jaws, and have their free extremities developed into 



laws ; 2. The possession of six abdominal pairs of appendages, 



