CHAPTER XX 

 PHYLUM ARTHROPODA (CONT'D) 



ARACHNIDA 



(By Vasco M. Tanner, Brigham Young University) 



Probably the most heterogeneous class of arthropods is the class 

 Arachnida, which is now a pigeon-hole for the spiders, scorpions, 

 mites, ticks, pseudoscorpions, harvestmen, whip scorpions, bear 

 animalcules, king crabs, and several other less common orders, such 

 as the Pentastomoids and Pycnogonids. 



In attempting to give the characteristics of this class we will list 

 only the obvious characters and then remind the reader that the mem- 

 bers of this group have more likenesses to the Arachnida than to any 

 other assemblage of animals, hence, no doubt, the reason for having 

 this class intact. 



The spiders and their relatives are characterized by having two 

 main divisions of the body, the cephalothorax and the abdomen; no 

 antennae or wings; mouth parts consisting of but two pairs of ap- 

 pendages, the chelicerae and pedipalpi; four pairs of walking legs, 

 each consisting of seven joints; lung books, which are remarkable 

 respiratory organs; simple eyes, that are definite in number and ar- 

 rangement in the various families ; abdominal appendages which have 

 been modified into spinning organs; and no metamorphosis, the de- 

 velopment being direct. 



Spiders 



The spider's body consists of two divisions: the cephalothorax and 

 abdomen. The first division of the body, the cephalothorax, results 

 from the fusion of the head and thorax. Six pairs of appendages 

 are to be found upon this part of the body. The first pair, the 

 chelicerae, are two-jointed, consisting of a basal part or mandible 

 and a terminal claw. The mandible is stout and covered on the in- 

 ner surface with small teeth and setae. The poison glands in the 

 mandibles discharge their poison through the movable sharp-pointed 

 claws. The poison serves as an aid to kill insects and other animals 

 used by the spider for food. The pedipalpi, consisting of six joints, 

 are located in front of the legs. They are used in handling the food, 



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