_Order 1._--_Gregarinida._ Body of a constant, usually elongate form, surrounded by a cuticle. In the early stage they lead an intracellular existence; in the mature stage they live within the intestine or body cavity of invertebrate animals, especially the Arthropoda, and, like intestinal parasites, are provided with clinging organs. Copulation usually isogamous; the spores have coats (chlamydospores) and usually contain several minute germs (sporozoites). _Order 2._--_Coccidiidea._ Body of uniform spherical or oval shape: they lead an intracellular life, but are not freely motile in cavities of the body. Fertilization is anisogamous; the spores have coats or shells (sporocysts), and usually contain several sporozoites. Exhibit alternation of generations. _Order 3._--_Hæmosporidia._ Parasites of the blood corpuscles of vertebrate animals; they exhibit amœboid movement; fertilization is anisogamous; many present alternation of generations and hosts; spores naked. _Sub-class 2._--*Neosporidia.* They are multinucleate when adult, and the form of the body varies exceedingly (often amœboid); spore-formation commences before the completion of growth. _Order 1._--_Myxosporidia._ The spores have valvular coats, with or without caudal appendages, with two, rarely four, polar capsules. They live free in such organs as the gall or urinary bladder, but are chiefly found in connective tissue. They occur especially in fishes. _Order 2._--_Microsporidia._ Spores with coats or sporocysts; no caudal appendage, with one polar capsule. They usually live in the tissues of Arthropoda. _Order 3._--_Sarcosporidia._ Elongate parasites of the muscular fibres of amniotic vertebrates, on rare occasions they occur also in the connective tissue; the spores, which are kidney or sickle-shaped, are naked and apparently have no obvious polar capsule. _Order 4._--_Haplosporidia._ Simple organisms, forming simple spores; they occur in Rotifers, Polychætes, Fish and Man. _Class IV._--*Infusoria* (_Ciliata_). The body is generally uniform in shape, with cilia and contractile vacuole, frequently also with cytostome; usually has macro- and micro-nucleus; live free in water and also parasitically. The orders _Holotricha_, _Heterotricha_, _Oligotricha_, _Hypotricha_ and _Peritricha_ are classified according to the arrangement of the cilia. _Class V._--*Suctoria.* Bodies with suctorial tubes, contractile vacuoles, macro- and micro-nucleus, no cytostome. They generally invade aquatic animals as cavity parasites, yet also attack plants; early stage ciliated. Live sometimes as parasites on Infusoria. [The Suctoria are frequently regarded as a sub-class of the Infusoria.]