ARTHROPODS. 



FIG. 9. Cephalopod (paper nautilus, female). 



The nervous system has three chief pairs of nerve centres 

 or ganglia. In many cases there are very characteristic 

 free-swimming larval stages. 



Arthropods.- -This large series includes Crustaceans, 



FIG. 10. Fresh-\\ atcr crayfish 

 (. Istaci/s], a Crustacean. 

 After Huxlev. 



FIG. it. a, Caterpillar; /', 

 pupa ; c, butterfly. 



