(3) The _Leucocytozoön_ type. The trophozoites and gametocytes occur within mononuclear leucocytes and young red cells (erythroblasts) in the blood of birds. Laveran and França consider that the Leucocytozoa occur in erythrocytes. The host cells are often greatly altered by the parasites, becoming hypertrophied and the ends usually drawn into horn-like processes (fig. 77), though some remain rounded. Leucocytozoa are limited to birds, and very rarely produce pigment. Male and female forms (gametocytes) are distinguishable in the blood (fig. 77), and the formation of male gametes (“exflagellation”) may occur in drawn blood. [Illustration: FIG. 77.--_Leucocytozoön lovati_. _a_, Male parasite (microgametocyte), within host cell, whose ends are drawn out; _b_, female parasite (macrogametocyte) from blood of grouse. × 1,800. (After Fantham.)] The Leucocytozoa were first seen by Danilewsky in 1884. They are usually oval or spherical. It is not easy sometimes to distinguish the altered host cell from the parasite, as the nucleus of the former is pushed to one side by the leucocytozoön. The cytoplasm of the female parasite stains deeply, and the nucleus is rather small, containing a karyosome. In the male the cytoplasm stains lightly and the nucleus is larger, with a loose, granular structure. Many species of Leucocytozoa are recorded, but schizogony has only been described by Fantham (1910)[191] in _L. lovati_ in the spleen of the grouse (_Lagopus scoticus_), and by Moldovan[192] (1913) in _L. ziemanni_ in the internal organs of screech-owls. [191] _Annals Trop. Med. and Parasitol._, iv, p. 255. [192] _Centralbl. f. Bakt._, Orig., lxxi, p. 66. M. and A. Leger[193] (1914) propose to classify Leucocytozoa, provisionally, according as the host cells are fusiform or rounded. [193] _Bull. Soc. Path. Exot._, vii, p. 437. (4) The _Hæmogregarina_ type. Included herein are many parasites of red blood corpuscles, with a few (the leucocytogregarines) parasitic in the white cells of certain mammals and a few birds. They are not amœboid but gregarine-like, vermicular or sausage-shaped (fig. 78). They do not produce pigment. They are widely distributed among the vertebrata, but are most numerous in cold-blooded vertebrates (fishes, amphibia and reptiles). The hæmogregarines of aquatic hosts are transmitted by leeches, those of terrestrial hosts by arthropods. The nucleus of hæmogregarines is usually near the middle of the parasite, but may be situated nearer one end. The body of the parasite may be lodged in a capsule (“cytocyst”). There is much variation in size and appearance among hæmogregarines. Some are small (_Lankesterella_); some attack the nucleus of the host cell (_Karyolysus_); others have full grown vermicules larger than the containing host corpuscle, and so the hæmogregarines bend on themselves in the form of *U* (fig. 78, _b_). Schizogony often occurs in the internal organs of the host, sometimes in the circulating blood.