Spiders and Their Near Relatives 



represented by less familiar animals, which also have the 

 body segmented and possess jointed legs. For an account of 

 the classes of arthropods not mentioned here, and for discussions 

 of the relations of the classes of arthropods to each other, the 

 reader is referred to more general works on zoology. 



I. Class Crustacea (Crus-ta'ce-a). — The more familiar 

 representatives of the Crustacea are the cray-fishes, the lobsters, 

 the shrimps, and the crabs. Cray-fishes (Fig. 3) abound in our 

 brooks, and are often improperly called crabs. The lobsters, 

 the shrimps, and the true crabs live in the sea. 



The Crustacea are essentially aquatic animals; a few of 

 them live in damp places on land, but most of them live in the 

 water. They breathe either through the general surface of the 

 body or by means of gills; they are never furnished with tracheae 

 as are nearly all other arthropods. 



The Crustacea differ also from all other arthropods in having 

 two pairs of antennas. In many of them the head and the thorax, 

 the part of the body which bears the ambulatory legs, are united, 

 forming a region known as the cephalothorax; the region behind 

 the cephalothorax is the abdomen. 



The examples named above are among the more conspicuous 

 members of the class; but many other smaller forms abound 

 both in the sea and in fresh water. Some of the more minute 

 fresh-water forms are almost sure to occur in any fresh-water 

 aquarium. In Fig. 4 are represented three of these, greatly 

 enlarged. 



Among the Crustacea that live in damp places on land are 

 the sow-bugs, Oniscidcc (O-nis'ci-dae). These frequently occur 

 about water-soaked wood. One of them is represented in Fig. 5. 



On the sea-coasts an immense number of species of Crustacea 

 occur. 



II. Class Diplopoda (Di-plop'o-da). — This class includes 

 the millipedes. These are air-breathing arthropods in which 

 the head is distinct, and the remaining segments of the body 

 form a continuous region (Fig. 6). The most striking char- 

 acteristic is the fact that most of the body-segments bear each 

 two pairs of legs. As a rule, the body is not flattened as with the 

 centipedes, and the antennae are comparatively short and few- 

 jointed. 



The millipedes live in damp places and feed on decaying 



