Invertebrates

a large group of animals lacking spines.

Invertebrates include Protozoa, sponges, Coelenterata, lower worms, mollusks, Articulata, echinoderms, and several other phyla—a total of 16 phyla of invertebrates. The division of animals into invertebrates and vertebrates was introduced in 1801 by the French biologist J. B. Lamarck. This division has no taxonomic significance. However, the term “invertebrates” is used descriptively in scientific and, especially, educational literature.

Invertebrates include the overwhelming majority of the animals that inhabit the earth. About 1.26 million species of invertebrates are known, while there is a total of 45,000 species of vertebrates. Insects are the most numerous invertebrates: more than 1 million species are known. (In reality, there are probably no fewer than 2 million species of insects.) Other groups of invertebrates with the number of species are Protozoa, 25,000 species; sponges, 5,000; Coelenterata, 9,000; lower worms, 20,000; mollusks, 107,000; and Articulata (excluding insects) no fewer than 79,000 species. Obviously, the number of species of invertebrates existing in nature is much higher. Every year several thousand previously unknown species are described.

Invertebrates are prevalent everywhere—in fresh water, in the seas and oceans, on land, and in the soil. Many are animal and plant parasites. The role of invertebrates in nature is very great. Solid remains of invertebrates that lived in previous geological eras are part of the composition of various sedimentary rocks. Sometimes these remains constitute the bulk of the rock. (For example, limestones consist almost entirely of the skeletons of extinct invertebrates —Foraminifera, corals, Bryozoa, mollusks, and others.)

The importance of invertebrates for mankind is great and varied. Many invertebrates or their products serve as food for humans (bees’ honey, crustaceans, mollusks, and others) or various game animals, birds, and fish. The products of the life activity of certain invertebrates have economic and industrial value (beeswax, silk threads from silkworms, shellac from Coccidia, dyes, such as sepia from cuttlefish, pearls and shells of mollusks, and skeletons of coral polyps). In a number of instances, invertebrates are used to fight harmful animals; that is, parasites and predators destroy the harmful animals. (This is the biological method of fighting pests that injure useful plants and animals.) In geology, the study of the remains of fossilized invertebrates is particularly valuable in determining the age of sedimentary rocks.

In addition to the useful invertebrates, there are many harmful ones, including animals that are carriers of infectious and parasitic pathogens; intermediate hosts of parasitic worms and carriers of communicable diseases; poisonous animals; pests that attack grain, grain products, and agricultural plants; and forest pests.