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 [. Zmetothoraca 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.) 
Orver 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 
of 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 Mitnz Epwarps, and espe- 
cially QuATREFAGES and Krogyer, refer them to the crustaceans. 
That some of these animals live parasitically on whales and other 
