Cephalothorax = unique to spiders; a combination of both the head and the thorax containing the legs, eyes, and mouthparts.
(Source: http://www.uky.edu/Ag/CritterFiles/casefile/spiders/anatomy/spideranatomy.htm)
Not significantly segmented into cephalothorax and abdomen
Distinctly segmented into cephalothorax and abdomen