Protozoa
The main subdivisions (called phyla) of the Protozoa are: phylum i. Sarcodina (Vol. 24, p. 208); phylum ii. Mastigophora (Vol. 17, p. 873); phylum iii. Sporozoa (Vol. 25, p. 734); phylum iv. Infusoria (Vol. 14, 557).
Metazoa
Coming next, the Metazoa in their order are, as follows: phylum i. Porifera (see Sponges, Vol. 25, p. 715); phylum ii. Hydromedusae or Hydrozoa (Vol. 14, pp. 135 and 171) which include aquatic animals of the coral kind; phylum iii. Scyphomedusa (Vol. 24, p. 519) which include groups of shell fish; phylum iv. Anthozoa (Vol. 2, p. 97) with the corals; phylum v. Ctenophora (Vol. 7, p. 592) including the jelly fish; phylum vi. Platyelmia (Vol. 21, p. 826) a group of animals in which creeping first became habitual; phylum vii. Nematoidea (see Nematoda, Vol. 19, p. 359) which include certain kinds of worms; phylum viii. Chaetognatha (Vol. 5, p. 789) an isolated class of transparent pelagic organisms; phylum ix. Nemertina (Vol. 19, p. 363) worm families; phylum x. Mollusca (Vol. 2, p. 669) shell-bearing animals.
Phylum xi. Appendiculata (Vol. 2, p. 220) which include the sub-phyla Rotifera (Vol. 23, p. 759), Chaetopoda (Vol. 5, p. 789), and Arthropoda (Vol. 2, p. 673), the sub-phylum which comprises practically the whole insect family. Important articles on animals in this class are: Hexapoda (Vol. 13, p. 418) which include the wasp, beetle, and other families; the Crustacea (Vol. 7, p. 552) which cover a field wide enough to embrace species as different outwardly as lobsters, wood-lice, and minute water fleas; and Arachnida (Vol. 2, p. 287) the spider family. Phylum xii. Echinoderma (Vol. 8, p. 871) with all the sea-urchins and star fish.
Phylum xiii. Vertebrata (Vol. 27, p. 1047) to which man belongs as an order of a sub-class of a class of a sub-phylum. The most important sub-phylum of the Vertebrata is the Craniata (see Vol. 27, p. 1048). The sub-phyla Hemichorda (Vol. 13, p. 257), Urochorda (see Tunicata, Vol. 27, p. 379), and Cephalochorda (see Amphioxus, Vol. 1, p. 886) deal with the lower orders of Vertebrata. The sub-phylum Craniata comprises the following classes: class i. Pisces, see Ichthyology (Vol. 14, p. 243) with the fishes; class ii. Batrachia (Vol. 3, p. 521), with the frog tribe; class iii. Reptilia (see Reptiles, Vol. 23, p. 136); and in close connection with this—class iv. Aves (see Bird, Vol. 3, p. 959, and Ornithology, Vol. 20, p. 299); class v. Mammalia (Vol. 17, p. 520) to which man belongs.
Phylum xiv. Mesozoa (Vol. 18, p. 187) minute parasitic animals intermediate between the Protozoa and the Metazoa. Phylum xv. Polyzoa (Vol. 22, p. 42) aquatic animals forming colonies by budding. Phylum xvi. Acanthocephala (Vol. 1, p. 109) including the parasitic worms. Phylum xvii. Podaxonia (Vol. 28, p. 1023), and phylum xviii. Gastrotricha (Vol. 11, p. 526) minute animals living at the bottom of ponds and marshes.
Natural History
This is an outline of the main division of the animal kingdom in their order as now classified. The subject of zoology is so vast that the student will probably confine himself to one branch of the subject, perhaps to one small fraction of a division, of which he proposes to investigate the complete natural history. As will be seen from the list below, which is classified, the Britannica offers an immense amount of material bearing on the subject. But of course the study of any one sub-class needs a general knowledge of the foundations of zoological science, so that some acquaintance with the principles on which the animal world is classified is indispensable. As in Botany, it will be easy to see from the article on any individual animal to which family it belongs so that the young student can work back from the particular to the general and find out the whole relationship of the subject in which he is interested by reference to the “systematic” article.