The life-cycle of a relatively simple gregarine, such as _Monocystis agilis_ (fig. 59), parasitic in earthworms, may now be considered. The gregarines, being members of the Sporozoa, produce spores at one phase of the life-cycle. Each gregarine spore (fig. 63) develops within itself a number of minute, sickle-shaped or vermicular bodies, known as sporozoites or primary infecting germs. Eight sporozoites are often formed within each spore. When absorbed by a new host, the spore softens and the sporozoites issue from it. They are capable of active movement and may or may not enter a cell, such as one of those of the digestive tract, or, as in _Monocystis_, a cell lining the vesicula seminalis which becomes a sperm-cell aggregate (sperm morula). When the sporozoite has reached the place of its choice in the host it ceases active movements and proceeds to feed passively on the fluid substances around it, whether they be those of tissues or body fluids. This passive, growing and feeding form is known as the trophozoite. After a trophic existence of longer or shorter duration, the trophozoite ceases to feed and prepares for reproduction. Two trophozoites associate together, each of them first becoming somewhat rounded. The two trophozoites, now known as sporonts or gametocytes, become invested in a single common envelope or cyst (fig. 64, _a_). The nucleus of each gametocyte then divides by a series of binary fissions (fig. 64, _b_), and the daughter nuclei thus produced arrange themselves at the periphery of the parent cells (fig. 64, _c_). Cytoplasm collects around each of these nuclei, and thus a number of gametes are formed within each gametocyte. The gametes for a time exhibit active movements, and ultimately ripe gametes of different parentage fuse in pairs, that is, conjugation occurs (fig. 64, _d_). In this way zygotes are produced, the nucleus of each zygote being formed by the fusion of two gamete nuclei. [Illustration: FIG. 64.--Schematic figures of conjugation and spore formation in Gregarines. For details see text. (After Calkins and Siedlecki, modified.)] [Illustration: FIG. 65.--_Stylorhynchus oblongatus_. _a_, cyst containing two sporonts or gametocytes, each full of gametes, those in the upper one being male. _b_, ripe male and female gametes. × 1,600. (After L. Léger.)] The zygote grows slightly and becomes oval or elongate, and at this period is often called the sporoblast. It then secretes an external membrane, the sporocyst. Nuclear division occurs inside the sporocyst by a series of three binary fissions (fig. 64, _e_), so that each sporocyst, now usually referred to as a spore, contains eight nuclei. The cytoplasm collects around each nucleus and eight vermicular sporozoites are produced within each spore (fig. 64, _f_), thus completing the life-cycle.