272 PRACTICAL LESSONS IN SCIENCE. 



The Arachnida include the ticks, mites, scorpions and spiders, 

 animals similar to the Crustacea, but differing from them in some 

 particulars. The arachnid a are air-breathing animals, having 

 only four pairs of legs, and usually no antennae. Almost all the 

 arachnida live on animals, a few only living on vegetable juices, 

 and we may expect to find agility and intelligence as character- 

 istics of this class. 



One group of this class are parasites in the nasal cavities of 

 dogs and wolves; the eggs find their way into the stomachs of 

 rabbits, penetrate to the liver and form a cyst ; after six months 

 or so they again become active for a time, and again encyst, in 

 which state they may reach the mouth of the original host. 



Another group including the mites and ticks are called the 

 Acarina. Their mouth apparatus is adapted for biting, or for 

 piercing and sucking. There are no circulatory organs, and the 

 nervous and respiratory organs are of the lowest order, suggest- 

 ing the idea that the class is made up of degraded, depauperate 

 forms, sunk to their present low estate through indolence and 

 high living. Itch-mites, cheese-mites, ticks, water-mites and oth- 

 ers belong to this group of repulsive animals. 



The Araneida, or true spiders, are distinguished by having a 

 much-swollen abdomen, four pairs of legs, an apparatus for spin- 

 ning webs, with from six to eight eyes, and generally poison 

 glands. In studying the spider make out the cephalothorax a.nd 

 abdomen, the mandibles, poison glands, maxillae, and the jointed 

 appendages of the maxillae, the maxillary palpi. Examine the 

 head for the eyes, usually from six to eight in number. Exam- 

 ine the posterior part of the abdomen for the spinnerets. Hold 

 a live spider by one leg with a pair of forceps and watch him 

 while spinning. The spinning glands secrete a viscid material 

 which, forced out through fine holes, hardens in the air to delicate 

 threads used by the spider for many purposes. Spiders differ 

 widely in form, color, and mode of life, but in any event they are 

 clean, active, intelligent and interesting. 



The Mygalidse are large spiders thickly covered with hair, who 

 dig holes in the earth, or appropriate holes already made in 



