CHAPTER XV 



PHYLUM AKTHROFODA— (continued) 



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

 and Pal;eostraca (King-crabs, E\irypterids, Trilo- 

 bites) 



The class Arachnoidea is far from being a coherent unity. 

 Its subdivisions are numerous and diverse, and a state- 

 ment 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 antennce 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 fewer. 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 cephalothorax. A plate-like internal skeleton, called 

 the endosternite, is often present. The elaborate compound 

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

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

 books [chambered trachece .?), or by both, or cutaneous, and 

 many would include the branchiate Palceostraca along with 

 Arachnoidea. In the tracheate 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. 



412 



