SECTION XIV 



THE EURYPTERID^E 



THIS last group of the Gigantostraca need not 

 detain us long. By the general consent of all the 

 zoologists who have recently studied these animals, 

 they are classed with the Xiphosuridae and the 

 Trilobites. The exact relationship, however, has not 

 hitherto been very clear ; we now find it in their 

 common origin from our Crustacean-Annelid. 



We have imagined our Crustacean-Annelid develop- 

 ing first of all a kind of crescent-shaped protection for its 

 bent head, arising primarily from the lateral projections 

 due to the bending of the cylindrical body. This 

 shield develops in almost every possible way. In the 

 Apodidae it forms a dorsal fold to cover the rest of 

 the cylindrical and unprotected Annelidan body ; in 

 the Ostracoda it forms the bivalve shell in a way to 

 be described later, or it gives rise, as described on p. 

 217, to the flat jointed dorsal roof extending over 

 the whole body in the Trilobites and the Xiphosuridae, 



