CHAPTER XV 
PHYLUM ARTHROPODA—(continued) 
Classes ARACHNOIDEA (Spiders, Scorpions, Mites, etc.) 
and PaL#ostTraca (King-crabs, Eurypterids, Trilobites) 
Tue class Arachnoidea is far from being a coherent unity. 
Its subdivisions are numerous and diverse, and a statement 
of general characters is consequently difficult. 
The anterior segments, about seven in number, are usually 
Jused into a cephalothorax, with six pairs of appendages. 
The most anterior of these 5 sible may be turned in front- 
of the mouth, but there are al antenne as in [nsects. 
The first two pairs of appendages (cheliceree and_pedipalps) 
generally have to do with seizing and holding the food ; the 
pthers are walking legs. But although six pairs occur in 
most, there may be more or less. 
but-not. always, without..appendages ; it may be segmented or 
unsegmented ; tt is generally distinct from, but may be fused 
to the cephalothorax. A plate-like internal skeleton, called 
the endosternite, ts often present. The elaborate compound 
eyes of Insects are notrepuescuted, the-eyes-being almost always 
spixation may be by tubular trachea, or..by lung: 
books (chambered trachee?), or by both, or cutaneous, and 
many would include the branchiate Paleostraca along with 
Arachnoidea. In the tracheate forms there are never more 
than four pairs of stigmata. Within all or some of the legs 
lie coxtaluglands,. perhaps comparable to nephridia. An 
elongated dorsal heart usually lies in the abdomen. The 
position of the genital aperture or apertures is usually on one 
of the anterior abdominal segments, All have, separate sexes, 
In most cases the newly hatched young are essentially like the 
adults—that 1s to say, there is no metamorphosis, 
