90 ARTHROPODA. 



Class II. Arachnida. 



Wingless Arthropoda with a cephalothorax, and four to six pairs of 

 legs. 



GROUP A. ARACHNIDA BRANCHIATA (Gigantostraca, Poecilopoda). 

 Aquatic gill-bearing Arachnidans. 



Order 1. Xiplwsura. A shield-shaped cephalothorax, with a single 

 pair of chelate appendages in front of the mouth, and five pairs of legs 

 with masticatory bases behind the mouth. The abdomen bears six pairs 

 of appendages, of which the first are united to form a genital operculum, 

 while the remainder are lamellar with numerous gill-folds. A spine- 

 like telson. Two large facetted frontal eyes, and two central dorsal 

 eyes. Mostly fossil. Limulus, first occurs in the lithographic slates 

 (Middle Oolite). L. polyphemus, recent. 



Order 2. Eurypterina. Extinct forms with small cephalothoracic shield, 

 one pair of praeoral appendages, and four pairs of legs with masticatory 

 bases around the mouth. Abdomen of twelve segments, devoid of 

 abdominal appendages (?), and ending in a broad telson. Eurypterus 

 pygmseus, Devonian. 



Order 3. Trilobita. Extinct forms with a cephalothoracic shield, no 

 prseoral appendages, but four pairs around the mouth. Abdomen bear- 

 ing appendages, and with a segmented anterior region. Calymene 

 Blumenbachii. Phacops. 



