70 



ZOOLOGY. 



thing except size. Tlie mites, however, have but three 

 pairs of legs in the larval state, and only obtain the 

 additional pair after casting the skin several times. 



90. Divisions. The Arachnida may be divided into 

 two sub -classes : 



1. Pulmonaria. These breathe by pulmonary sacs, and 

 sometimes by tracheae also. They have six or eight 

 simple eyes. The scorpions and most spiders belong to 

 this sub-class. 



2. Trachearia. The members of this group breathe 

 either by tracheae or by the general surface of the body. 

 They have never more than four simple eyes. This sub- 

 class includes the cheese-mite, itch-insect, and harvest- 

 spider. 



CLASS 4. Crustacea. 



91. The class Crustacea (Lat. crusta, a crust) is so called, 

 because the body is often covered by a skin which con- 

 tains a considerable quantity of carbonate of lime, and 

 is hardened into a shell or crust. This skin forms the 

 external skeleton, and within it all the soft parts of the 

 body are inclosed. The Crustacea is a very large class, 

 including lobsters, crabs, shrimps, prawns, barnacles, sea 

 acorns, &c., groups of animals that differ widely in ap- 

 pearance and structure. It is closely allied to the 

 Arachnida ; but may be distinguished from it and 

 the other classes of Arthropoda by the following 

 characters : When breathing organs are present, they 

 always take the form of gills or branchiae, which adapt 

 them for respiring air in water ; they have always more 

 than four pairs of limbs (the usual number being five or 

 seven pairs), which are not confined to the thorax, but are 

 usually attached to the abdomen also ; one or more of 

 these pairs of limbs are usually modified into masticating 

 jaws ; there are two pairs of antennae ; they undergo 

 metamorphosis before arriving at maturity. 



92. Skeleton. The body is usually divided into three 



