INTRODUCTORY 3 



a rough guide to classification, it is useful to remember 

 that an Insect "can generally be recognized by having 

 three pairs of walking legs, an Arachnid by having 

 four pairs, and a Centipede or Millipede by having a 

 great many pairs, all nearly alike. The Crustacea, 

 on the other hand, show great diversity in the number 

 and arrangement of their walking or swimming legs, 

 but they rarely show any special resemblance to those 

 of the other large groups of Arthropoda. Thus, for 

 example, a common species of Woodlouse, Armadil- 

 lidium vulgare, is very similar at first sight to the 

 Millipede Glomeris marginata, but it has only seven 

 pairs of walking legs, while the Millipede has seven- 

 teen or nineteen pairs. 



More precisely, it may be said of the great majority 

 of Crustacea that they are aquatic animals, breathing 

 by gills or by the general surface of the body, having 

 two pairs of " feelers," or antennae, on the front part 

 of the head, and at least three pairs of jaws. Excep- 

 tions to each of these statements will be mentioned 

 in later chapters in dealing with parasites and other 

 highly modified types. In such cases, however, the 

 larval or young stages afford indications of affinity, 

 and comparison with less modified forms enables us 

 to trace a connection with the typical Crustacea. 



The best way to form a conception of a group ol 

 animals, however, is not to attempt in the first place 

 to define its limits, but to begin by studying the 

 structure of some typical and central species, and 



