ARTHROPODA 123 



lunglike sacs, from two to four in number, situated under 

 the abdomen. All the species are carnivorous. 



The instincts of spiders are of a high order. They 

 are, perhaps, the most wily of arthropods. They display 

 remarkable skill and industry in the 

 construction of their webs ; and some 

 species (called "mason spiders") even 

 excavate a subterranean pit, line it with 

 their silken tapestry, and close the en- 

 trance with a lid which moves upon a 



, . n\ ritf. 02. spinnerets 



hinge/* of the spider, b,c; a, 



4. Acarida, represented by the mites palpiform organs ' 

 and ticks. They have an ov-al or rounded body, without 

 any marked articulations, the head, thorax, and abdomen 

 being apparently merged into one. They have no brain ; 



only a single ganglion 

 lodged in the abdomen. 

 They breathe by tracheae 

 FIG. 8 3 . -A Mite ( D em o d exfoinc Jt io- through the skin. The 



rum), one of the lowest Arachnids; a niOUth is formed for SUC- 

 parasite in human hair sacs ; X 125. 



tion, and they are generally 



parasitic. The mites (Sarcoptes) are among the lowest 

 of articulates. The body is soft and minute. The ticks 

 (Ixodes) have a leathery skin, and are sometimes half 

 an inch long. The mouth is furnished with a beak 

 for piercing the animal it infests. 



5. Xiphosura, Arachnida with a broad carapace cov- 

 ering the cephalothorax, an abdomen consisting of seven 

 firmly united segments ending with a long slender tail 

 of one piece, five pairs of legs on the cephalothorax; 

 the abdomen with five pairs of platelike respiratory 

 organs covered anteriorly by an operculum. The king 

 crab or horseshoe crab (Limtilus), found on muddy 

 bottoms along the coast, belongs in this order, which is 

 interesting as containing the only living representatives 



