v. 21. Simple and wormlike animals -- v. 22. Insects 1 : Millipedes and unusual insects -- v. 23. Insects 2 : Crickets, grasshoppers, and flies -- v. 24. Insects 3 : True bugs -- v. 25. Insects 4 : Beetles -- v. 26. Insects 5 : Butterflies and moths -- v. 27. Insects 6 : Wasps, ants, and bees -- v. 28. Crustaceans -- v. 29. Arachnids -- v. 30. Mollusks and echinoderms.