46 PRIMITIVE ANIMALS 



still exists on the Pacific Coasts of America and 

 Japan. This animal, the KING-CRAB or LIMULUS 

 (Fig. 8), owing to its outward appearance and to its 

 being an inhabitant of the sea, was at first considered 

 to be a Crustacean, but an examination of its anatomy 

 has shown it to be undoubtedly an Arachnid. It 

 grows to a large size, nearly two feet in length, 

 but Palaeozoic fossils (Eurypterida), related to the 

 King-Crab, attained the immense length of six feet. 

 These creatures, looking like gigantic scorpions, 

 apparently lived at slight depths near the sea- 

 shores like Limulus, but what their prey was or 

 for what purpose they attained their immense size 

 is not known. That the King-Crab is an Arachnid 

 is conclusively shown by the structure of its eyes 

 and by that of the anterior pair of limbs which 

 are not antennae but pincers as in the Spiders, but 

 the gills, which are carried externally upon the 

 posterior limbs, are folded vascular plates which 

 recall to some extent the gills of Crustacea. The 

 terrestrial air-breathing Arachnid most nearly related 

 to the King-Crab is the Scorpion, and this animal has 

 peculiar respiratory organs, known as lung-books, 

 which are situated on the same segments as the gills 

 of Limulus, but they are sunk below the skin and 

 open to the exterior by small orifices, the stigmata. 

 It is practically certain that the Spiders have acquired 

 their chief breathing organs from such lung-books, in 



