Fig. 62.—Gregarina munieri (from the beetle, Chrysomela hæmoptera). Section through surface layers. Cu, cuticle; E, ectoplasm proper; G, gelatinous layer; My, myonemes in myocyte layer. × 1500. (After Schewiakoff.)
The endoplasm is fluid and granular, containing many enclosures, which are of the nature of reserve food materials. They consist of fat droplets or of paraglycogen, and give the organisms an opaque appearance. Lithocystis contains crystals of calcium oxalate in its endoplasm.
Many gregarines are capable of active movements, though they do not possess obvious locomotor organs. The movement is of a smooth, gliding character and two suggestions have been put forward to explain it. According to Schewiakoff, a gelatinous substance is secreted between the layers of the ectoplasm. This is extruded posteriorly and thus the animal is pushed forward. On the other hand, Crawley considers that the movements are produced by contractions of the myonemes. These two explanations are probably correct as far as each goes, and are to be regarded as supplementary to one another.
Occasionally, temporary associations of gregarines are formed by a number of individuals adhering to one another end to end. Such temporary associations are examples of syzygy. Such syzygies must not be confused with true associations which form an essential part of the life-cycle.
Fig. 63.—Monocystis agilis. Spores from vesicula seminalis of the Earthworm. a, Sporoblast with single nucleus, enclosed in sporocyst; b, mature spore containing sporozoites; c, diagrammatic cross-section of spore, showing eight sporozoites round residual protoplasm. (After Bütschli.)
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.