CHAPTEE XX 



TEACHEATA 



ARACHNIDA 



ARACHNIDA-i 



Beanohiate 



FOKMS 



Tkacheate 



FOKMS 



Abekkant 

 Forms. 



C Eurypterina — Eurypterus, Pterygotus. 

 J. Trilobita — Olenus. 

 \ Xiphosura — Limulus. 



Scorpionida — Scorpio, Buthus. 

 Pseudoscorpionida — Chelifer, OMsium. 

 Pedipalpi — Phrynus, Thelyphonus. 

 Solifuga — Galeodes- 



iTetrapneumones — Mygale, 

 Atypus. 

 Dipneumones— ^cira, 

 Zycosa. 

 Phalangida — Phalangium, Gonyleptus. 

 .Acarina — Ixodes, Sar copies. 



i LinguatuKda. 

 -] Tardigrada. 

 (. Pycnogonida. 



Characteristics. — Arthropoda with a ce^jlialothorax (prosoma) 

 marked off from the rest of the hody hy the presence of a carapace 

 and by the character of its lirtibs. Tlie abdomen may he segmented 

 or unsegmented, and is sometimes divided into two regions, 

 the mesosoma and the metasoma, the latter may bear a posterior 

 spine. Four pairs of walking-legs are usually present. The 

 breathing! apparaftis may take the form of branchiae adapted 

 for aquatic respiration, or of lung-books, or tracheae for 

 breathing air. An internal skeleton in the form of a plate, 

 termed the endosternite, is present in many forms. The 

 genital orifice is usually on one of the anterior segments of the 

 abdomen. They are, vjith the exception of one order, bisexual. 

 The Arachnida fall into three groups — those that breathe 

 water, those that breathe air, and certain aberrant forms, — 

 and the members of groups differ to a very considerable extent. 



