274 



ARTHROPODA 



[CH. 



its posterior surface a gill-book. There is a striking similarity 

 between these organs and the " lung-books " of the Scorpion ; 

 but the gill-books are not sunk in pits. The metasoma terminates 

 at the anus, but behind it a long sword-like tail projects. This 



FIG. 125. Ventral view of the King-crab, Limulus potyphemus x ^. 



1. Carapace covering prosoma. 2. Meso- and meta-soma. 3. Telson. 

 4. Chelicera. 5. Pedipalp. 6, 7, 8, 9. 3rd to 6th appendages, 



ambulatory limbs. 10. Genital operculum turned forward to show the 

 genital aperture. 11, 12, 13, 14, 15. Appendages bearing gill-books. 



16. Anus. 17. Mouth. 18. Chilaria. 



post- anal tail corresponds with the swollen stinging tail or telson 

 of the Scorpion. It is used by the animal to right itself when it 

 is upset by the motion of the waves. 



A curious plate of fibro-cartilage to which muscles are attached 



