_Monocystis_ offers us the simplest type of GREGARINIDACEAE. In most Gregarines (Figs. 31, 32) the sporozoite enters the epithelium-cell of the gut of an Arthropod, Worm or Mollusc, and as it enlarges protrudes the greater part of its bulk into the lumen, and may become free therein, or pass into the coelom. The attached part is often enlarged into a sort of grapple armed with spines, the "epimerite"; this contains only sarcocyte, the other layers being absent. The freely projecting body is usually divided by an ingrowth of the myocyte into a front segment ("protomerite"), and a rear one ("deutomerite"), with the nucleus usually in the latter. In this state the cell is termed a "cephalont." Conjugation is frequent, but apparently is not always connected with {99}syngamy or spore-formation; sometimes from two to five may be aggregated into a chain or "syzygy." The number of cases in which a syngamic process between two cells has been observed is constantly being increased. In _Stylorhynchus_ (Fig. 33) the conjugation at first resembles that of _Monocystis_, but the actual pairing-cells are bisexually differentiated into sperms in the one parent, and oospheres in the other; it is remarkable that here the pear-shaped sperms are apparently larger than the oospheres. In _Pterocephalus_ the chief difference is that the sperms are minute.[106] In all cases of spore-formation the epimerite is lost and the septum disappears; in this state the cell is termed a sporont. Sometimes the epiplasm of the sporont forms tubes ("sporoducts"), which project through the cyst-wall and give exit to the spores, as in _Gregarina_ (Fig. 32, C), a parasite in the beetle _Blaps_. Gregarines infest most groups of Invertebrates except Sponges and perhaps Coelenterates, the only exception cited being that of _Epizoanthus glacialis_, a Zoantharian (p. 406). They appear to be relatively harmless and are not known to induce epidemics.