IN SEC 7' S AND THEIR NEAR RELATIVES. II 



B. Without antennae and with four pairs of legs, although the 

 maxillary palpi are often leg-like in form, making the animal 



appear to have five pairs of legs. p. 12 Arachnida. 



BB. With antennae. 

 C. With more than three pairs of legs ; and without wings, p. 



45 Myriapoda. 



CC. With only three pairs of legs, and usually with wings in 

 the adult state, p.48 Hexapoda. 



Class Crustacea (Crus-ta'ce-a). 

 The Crustaceans {Crus-ta ' ce-ans). 



The members of this class are aquatic Arthropoda, which 

 breathe by true gills. They have two pairs of antennce and at 

 least five pairs of legs. 



The most familiar illustrations 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 salt water. 



The Crustaceans are 

 distinguished from all oth- 

 er Arthropods by their 

 mode of respiration, being 

 the only ones that breathe 

 by true gills. Many in- 

 sects live in water, and 

 are furnished with gill-like 



- . Fig. 3 —A Cray-fish. 



organs ; but these are 



tracheal gills, organs which differ essentially in structure 

 from true gills, as described later, in the chapter on Anat- 

 omy of Insects. The Crustacea also differ from other 

 Arthropoda in having two pairs of antennae ; and from all 



