GIGAtfTOSTKACA MEROSTOMATA. 479 



Gela$inni8 forceps Latr., Grapsus varlus Latr., Gecarcinus rurlcola L., Land 

 Crabs. Water is retained for a long time in the branchial cavities, owing to the 

 presence of secondary spaces around the branchial plates, which are thus pre- 

 vented from sticking together. They live in holes in the earth in the Antilles. 



III. GIGANTOSTKACA. 



The Xiphosura or Pcecilopoda, represented by the living genus 

 Limulus and the orders of the fossil Merostomata, may be united 

 under this head, as opposed to the Entomostraca and Malacostraca. 



They are principally characterised by the possession of a single 

 pair of appendages placed in front of the mouth and innervated from 

 the cerebral ganglion, also by the presence of four or five pairs of 

 legs, which are placed round the mouth and whose basal joints are 

 modified to form large mandible-like masticatory organs. Behind 

 the last pair of legs there is a simple or cleft prominence, forming 

 a sort of underlip. The region of the body which bears these appen- 

 dages is to be considered as an unsegmented cephalo- thorax ; it is 

 shield-shaped and may be drawn out into projecting wing- shaped 

 lateral portions. On its upper surface two small median frontal eyes 

 as well asttwo large lateral eyes can be distinguished. Following the 

 cephalo -thorax there is an abdomen, which is usually elongated and 

 composed of a greater number of segments. The abdomen tapers 

 posteriorly and terminates in a telson, which may be flat or drawn 

 out into the form of a spine. 



Order 1. MEROSTOMATA.* 



Gigantostraca 'with five pairs of appendages on the, ceplialo-tliorax 

 which is relatively short ; with an elongated apodal abdomen, usually 

 composed of twelve segments and ending in aflat or styliform telson. 



The powerful body of the Eurypteridce (included with the Pcecilo- 

 poda by "Woodward), as the most important family of the Merostomata 

 is named after the genus Eurypterus, consists of a cephalo-thoracic 

 shield with median ocelli as well as large projecting marginal eyes, also 

 of an abdomen with numerous (usually twelve) segments which become 

 longer posteriorly, and of a caudal shield, which is prolonged into a 

 spine. Round the mouth on the underside of the cephalo- thorax 



* Woodward, " Monograph of the Brit, fossil Crustacea belonging to the 

 order of Merostomata." P. I., % II., Palceont. Soc. of London, 1866-1869. Wood- 

 ward, " On some points in the structure of the Xiphosura, having reference to 

 their relationship with the Eurypteridas," Quarterly Jour*. Geol.Soc. of London, 

 1867 and 1871. 



