Organotherapy, or the treatment of disease by means of animal secreting organs or of preparations derived from them, has been continually in use for some 5,000 years, but it has recently been scientifically studied and considerably extended. Before proceeding to this section of the work it will be desirable to take a very brief survey of the animal kingdom, more especially as several animal substances have already been described.

Various systems of zoological classification have been from time to time proposed, but the following one is readily intelligible and well adapted for the object in view. By it the animal kingdom is divided successively into Phyla, Classes, Orders, Families, Genera, and Species. The following outline embraces the chief Phyla, Classes, and Orders, and indicates their most important characters.

Phylum I

Protozoa. This phylum includes the simplest and most primitive animals, each consisting of a minute, single cell. Loose colonies are sometimes formed by division or budding, but differentiated tissues are not developed. Most of them live in water, and some form calcareous or siliceous skeletons. They are grouped into three classes, viz. the Rhizopoda, which are predominantly amoeboid, the Sporozoa, which are usually encysted, and the Infusoria, which are actively motile by means of cilia or flagella. The Rhizopoda include such animals as the Foraminifera, the accumulated calcareous skeletons of which form most chalks and limestones. The Sporozoa are mostly parasitic, e.g. the malarial parasite. The Infusoria move actively and are found in great numbers in stagnant water, vegetable infusions, etc.

Phylum II

Porifera. These are mostly passive, vegetative, marine animals which form colonies and exhibit slight division of labour, sexuality, etc. Familiar examples are the ordinary sponges in which very minute pores on the surface (inhalant canals) lead into chambers lined with flagellated cells. The movement of the flagella draws a current of water with food particles through the sponge, the waste products being discharged through larger exhalant openings. The Porifera have no body cavity, but often secrete a horny (bath sponge), calcareous, or siliceous skeleton.

Phylum III

Coelentera. Multicellular animals which, like the Porifera, have no body cavity; they are usually radially symmetrical and develop stinging cells. The most familiar examples are the sea-anemones, jelly-fishes, and corals, the last-named being colonies of individuals which secrete a calcareous skeleton.

Phylum IV

Platyhelminthes or Flat-worms. In this group the primitive radial symmetry is lost; the body is flattened but no true body cavity is formed. Here the first indications of a head and brain are to be discerned. To this phylum belong the Trematodes or Flukes (liver fluke of the sheep) and the Cestodes or Tapeworms, all of which are parasitic.

Phylum V

Nemertea or Ribbon Worms. These are the first animals possessing an open gut and closed blood-system; they are mostly marine and, for the purpose in hand, unimportant.

Phylum VI

Nematohelminthes. Thread-like worms, mostly parasitic and provided with a distinct body cavity, a well-developed alimentary canal, a mouth, and an anus. To this phylum belong such well-known parasites as the round-worm (Ascaris), the threadworm (Oxyuris), the Guinea worm (Dracunculus), Trichina, etc.

Phylum VII

Annelida. These exhibit a well-developed coelom and a distinct segmentation of body which is visible externally. The Annelida include the Chaetopoda or worms with bristles (earth-worm, lob-worm) and the Hirudinea or worms devoid of bristles (leech).

Phylum VIII

Echinoderma. The echinoderms are radially symmetrical animals which exhibit a tendency to form calcareous skeletons. Familiar examples may be found in the sea-cucumbers, sea-urchins, star-fishes, etc.

Phylum IX

Arthropoda. This important phylum consists of bilaterally symmetrical animals exhibiting numerous segments and bearing paired, jointed appendages; the sexes are always separate. The chief classes are Crustacea, Myriopoda, Insecta, and Arachnoidea.

The Crustaceans include the crabs, lobsters, cray-fish, shrimps, etc.; they mostly live in water and breathe by gills.

The Myriopods embrace the centipedes, millipedes, etc.

The Insects are usually winged, breathe by a system of air-tubes and undergo a metamorphosis. They are subdivided into eighteen orders of which the following are the most important for the purposes of the present work.

(A) Hymenoptera

Ants, bees, wasps, etc.; they have four transparent wings, are usually provided with mandibles, and are furnished with a sting or with an ovipositor.

(B) Lepidoptera

Butterflies and moths; they have four scaly wings, a suctorial mouth, and no ovipositor.

(C) Diptera

House-flies, gnats, midges, etc.; they have one pair of transparent wings and a suctorial mouth; the larva is a maggot.

(D) Coleoptera

Beetles; these have one pair of wings and one pair of hardened wing-cases; they are provided with biting mandibles.

All the members of the foregoing orders undergo a complete metamorphosis from larva to perfect insect.

(E) Hemiptera

Aphides, Coccus insects, bugs, etc.; these have piercing, suctorial mouths and either two pairs of wings or none (the male Coccus insect alone undergoes complete metamorphosis).

The Arachnoidea include the scorpions, spiders, mites, ticks, etc.

Phylum X

Mollusca. The Molluscs (snails, mussels, oysters, cuttlefishes, etc.) are unsegmented, have no appendages, and have a soft body secreting an outer shell (in the cuttlefish an inner, calcareous shell).

Phylum XI

Chordata (Verlebrata). These have a dorsal supporting axis and a dorsal, tubular nervous system. The most important classes are the Pisces, the Amphibia, the Reptilia, the Aves, and the Mammalia.

The chief sub-classes of the Pisces are (i) Elasmobranchii or Cartilaginous Fishes such as the skate, shark, dog-fish, etc, and (ii) Teleostomi, which have a more or less bony skeleton. The most important order of Teleostomi is Teleostei, or true Bony Fishes, which includes the cod, herring, salmon, and most of the modern fishes.

The class Amphibia are those vertebrates which exhibit a transition from aquatic to terrestrial life. The frog, toad, and newt are types.

The class Reptilia includes the tortoises, turtles, lizards, snakes, alligators, etc.

The class Aves includes the divisions Ratitoe or Running Birds (ostrich) and the Carinatoe or Flying Birds.

Of the class Mammalia the following Orders belonging to Sub-class Eutheria or placental animals may be mentioned: (i) Xenarthra (sloths, anteaters); (ii) Nomarthra (pangolins); (iii) Sirenia (dugong, sea-cow); (iv) Ungulata (hoofed animals such as the ox, hog, sheep, musk-deer, etc.); (v) Cetacea (whales, dolphins, etc.); (vi) Rodentia (rats, beavers, etc.); (vii), Carnivora (lions, tigers, bears, etc.); (viii) Pinnipedia (seals, walruses); (ix) Insectivora (hedgehogs, shrews, etc.); (x) Chiroptera (bats); (xi) Prosimioe (lemurs); (xii) Anthropoidea (monkeys, baboons, apes, man).