300 MANUAL OF ZOOLOGY. 



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, ttpon 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 metas- 

 toma 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 appendages ; the telson, or terminal segment, 

 ensiform " (Henry Woodward). 



The Xiphosura include no other recent forms than the 

 Limuli (King-crabs, or Horse-shoe Crabs). They are dis- 

 tinguished by the possession of six pairs of chelate limbs, placed 

 round the mouth, having their bases spinous, and officiating as 

 jaws. The anterior portion of the body is covered by a broad 

 horse-shoe-shaped buckler (fig. 150), the upper surface of 

 which bears a pair of larval and a pair of compound eyes. 

 On the lower surface of the carapace is placed the aperture 

 of the mouth, surrounded by six pairs of limbs, the bases of 

 which are spinous and officiate as jaws, whilst their termina- 

 tions are converted into chelae or nipping-claws. The first 

 pair of appendages is placed in front of the mouth, and has 

 been generally said to represent the antennae ; but according 

 to Milne-Edwards they are not supplied with nerves from the 

 cerebral ganglia, and therefore cannot be of this nature. Be- 

 hind the cephalic buckler comes a second shield, composed 

 of six amalgamated segments, below which are carried the re- 

 productive organs and branchiae, the former protected by a 

 thoracic plate or " operculum," the latter borne by five pairs 

 of lamellar appendages. Lastly, articulated to the posterior 

 margin of the abdominal shield, is a long sword-like spine or 

 "telson" (fig. 150, /). The circulatory system of Limulus is 

 of a very high type, though the heart is tubular. The venous 

 blood, instead of being contained in the mere interspaces and 

 lacunae between the tissues, is to a large extent confined within 

 proper vessels. A remarkable peculiarity, also, is that the 

 ventral nerve-cord is enclosed within the abdominal artery, and 

 most of the nerves are similarly ensheathed within the arteries. 



The eggs of Limulus are laid in the sand, and are fertilised 

 by the male. Just prior to the time of hatching, six segments 

 can be recognised in the cephalothorax ; the abdomen consists 

 of nine well-marked somites ; the bases of the legs are hardly 

 spinose ; and the abdominal spine is quite rudimentary. In 

 this stage (fig. 152), the larva closely resembles some of the 



