CRUSTACEANS 6 1 



the mercy of birds, fish, and other enemies while in this 

 soft and defenceless condition, it stays hidden until the 

 covering hardens. Hence it cannot eat much, but probably 

 by the absorption of water the tissues grow ; that is, enlarge. 

 In the intervening periods, when growth is impossible, it 

 develops ; that is, the tissues and organs change in structure 

 and become stronger. " Soft-shelled crab " is a popular dish, 

 but there is no species by that name, this being only a crab 

 just after moulting which has been found by fishermen in 

 spite of its hiding. 



General Questions. — How do crayfish choose their food? 

 How long can they live out of water? Why do their gills remain 

 moist out of water longer than a fish? How do they breathe 

 out of water? Are they courageous or. cowardly animals? When 

 they lose appendages in fighting or moulting these are readily 

 reproduced, but an organ moults several times in regaining its 

 size. Have you seen crayfish with one claw smaller than the 

 Compare the crayfish and crab (Figs. 81, 93, and 95) in the 

 following particulars : shape, body, eyes, legs, abdomen, habitat, 

 movement. 



KEY TO THE FOUR CLASSES IN BRANCH ARTHROPODS 



i. Insects . . . 3 body divisions, 6 legs 



2. Arachnids . . 2 body divisions, 8 legs 



3. Myriapods . . many body divisions, many legs 



4. Crustaceans . gill breathers, skeleton (external) limy 



By the aid of the key and of figures 96-105, classify the following 

 Arthropods : tick, thousand-leg centipede, king crab, pill bug, spider 5 

 scorpion, beetle. 



