SYSTEMATIC 

 ARRANGEMENT OF ARACHNIDS. 



CLASS IX. 

 ARACHNOIDEA. 



ARTICULATE animals with articulate feet. Head and thorax 

 conjoined to form a single part. Feet eight, placed at the sides of 

 cephalothorax; abdominal feet none. Heart placed in the back, 

 resembling an elongate vessel, in many giving off arteries. Respi- 

 ration in some tracheal, in others pulmonal ; in some no distinct 

 organs of respiration. Sexes mostly distinct. 



Section I. Tmetothoraca s. Apneusta. Cephalothorax divided 

 into four segments. Stigmata none. Organs of respiration none. 

 (Seat of respiration either in the external integument of body or in 

 the digestive canal.) 



ORDER I. Polygonopoda. 



Feet elongate, of the length of body or longer than body. First 

 .segment of body tubular, exsert, perforated at the apex by the 

 mouth. Ocelli four in a tubercle behind the tube at the middle part 

 rof the second segment. Abdomen small, conical. 



Family I. Pycnogonida (characters of the order). 



Sea-spiders. The genus Pycnogonum of BRUENNICH (Polygonopus 

 PALLAS), with some other allied genera of later writers, forms a 

 small group of marine animals, on the true place of which in the 

 natural arrangement opinions differ ; for MILNE EDWARDS, and espe- 

 cially QUATREFAGES and KROEYER, refer them to the crustaceans. 

 That some of these animals live parasitically on whales and other 



