CHAPTER XV. 



ARACHNOIDEA AND PAL^OSTRACA. 



Class Arachnoidea. Spiders, Scorpions, Mites, etc. 



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 fused into 

 a cephalothorax, which bears six pairs of appendages. The 

 most anterior of these appendages may be turned in front of 

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

 The first two pairs of appendages (chelicera, 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 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 trachea, or by lung- 

 books (chambered trachea ?), or by both, and many would 

 include the branchiate Palaostraca 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. The relation 

 of Arachnoids to other Arthropods, and especially to Insects, is 

 uncertain. Many believe that the Arachnoids are descended 



