ANNULOSA: CRUSTACEA. 221 



glabella — in which case the free cheeks will form a single piece— or they 

 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 

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

 "axis," and of two flatter side-lobes, called the "pleurse." 



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 

 furnished with mandibles and maxillae, the terminations of 

 which become walking., or swimming feet, and organs of pre- 

 hension. 



This order comprises the recent King Crabs, and the extinct 

 Pterygoti and Eurypteri. 



Sub-order i. Xiphosura.- — " Crustacea having the anterior 

 segments welded together to- form a broad convex buckler, 

 upon the dorsal surface of which are placed the compound . 

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

 centre in front. The mouth is furnished with a small labrum, 

 a rudimentary metastoma and six pairs of appendages. Pos- 

 terior 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 Horse-shoe Crabs) (fig. 69). They 

 are distinguished by the possession oi 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 branchiEe. 

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

 exhibits a division into three portions : — (i) An anterior semi- 

 circular shield, which cames two compound and two simple 

 eyes ;. (2) a posterior, irregularly hexagonal shield, whi«h 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 : — ^i. The possession of six pairs of appendages 

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



