XIPHOSURA 



421 



The caudal spine is still a very short and simple plate. The 2 compound eyes 

 and the 2 ocelli are already present. Behind the 1st pair of abdominal feet the 

 rudiments of the 2d are visible. At this stage the larva has the appearance 

 of a TriloUtc, and this similarity is increased by two dorsal longitudinal furrows. 

 The gradual transition from the Trilohite stage to the Limulus stage is brought about 

 by the appearance in order from before backward of the abdominal leaf-like feet. The 



Fig. 2S7.— Limulus pol3rpli6nius in tlie so-oalled Triloliite stage. A, Dorsal side ; 

 B, ventral side (after Kingsley). 



leaf-like feet become biramose. Gills develop on them ; in the Trilohite stage rudi- 

 ments of gills were found only on the most anterior pair of abdominal feet. The 

 abdomen loses its segmentation. The caudal plate elongates by degrees into the 

 caudal spine. 



Systematic position. — The relationship of the Xiphosura to the Crustacea is in 

 any case very distant, since it is at present impossible to homologise the extremities 

 of Zimtilus -with typical Crustacean extremities, or to compare in detail the segmenta- 

 tion of Limulus with that of any Crustacean. The biramose character of the leaf-like 

 feet and of the 6th pair of thoracic feet is the only specific Crustacean characteristic 

 shown by the XiiiJiosura, if we leave the gills out of consideration. The relation 

 between the Xijihosiira and the fossil Heniiaspidce and Gicjantostraca is evidently 

 much closer. The Xiphosura, ffemiccspidce, and Giycmtostraca are themselves again 

 perhaps racially connected with the Trilobites. In any case, however, in the present 

 state of science, it seems probable that all these groups are only connected at their 

 roots with the Crustacea. The relations of the Gigantostraca and Xi2]hosura to the 

 Arachnoidea, especially to the Scorpions, which is assumed by some observers, will 

 be discussed later. 



Single genus Limulus. — Marine, L. nwluccanus, Molucca, Sunda Islands ; L. 

 polyphemMs, East coast of North America. 



Most Important Literature. 



For general guidance. 



Karl A. Zittel. Handbuch dcr Palceontologie. 1. Abth. 2. Band. Mollusm und 

 Arthropoda. Miinchen und Leipzig, 1881-1885. 



Trilobitse. 

 H. Burmeister. Die Organisation der Triloiiten. Berlin, 1843. 



