CHAPTER XV. 



PHYLUM ARTHROPODA (contitnted). 



Classes ARACHNOIDEA (Spiders, Scorpions, Mites etc.), 

 and PAL^:OSTRACA, (King-crabs, Eurypterids, Trilobites). 



THE 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 

 fused into a cephalothorax, with six pairs of appendages. The 

 most anterior of these appendages may be turned in front of 

 the mouth, but there are no pre-oral antenna as in Insects. 

 The first two pairs of appendages (chelicerce and pedipalps] 

 generally have to do with seizing and holding the food ; the 

 others are walking legs. But although six pairs occur in 

 most, there may be more or less. The abdomen is generally, 

 but not always, without appendages ; it may be segmented or 

 unsegmented ; it is generally distinct from, but may be fused 

 to the cephatothorax. A plate-like internal skeleton, called 

 the endosternite, is often present. The elaborate compound 

 eyes of Insects are not represented, the eyes being almost alwavs 

 simple. Respiration may be by tubular trachece, or by lung- 

 books (chambered trachecel}, or by both, or cutaneous, and manv 

 would include the branchiate Palceostraca along with Arach- 

 noidea. In the trachea te forms there are never more than 

 four pairs of stigmata. Within all or some of the legs lie 

 coxal glands, 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 is 

 to say, there is no metamorphosis. 



