The cycle of development is usually completed in a single host; in some instances in its faeces, as in Coccidium oviforme, the intestinal Coccidia of the Rabbit and birds and in Cyclospora from Glomeris1.

As to other Gregarinida. The Monocystids of the Earthworm quit the sperm-blastophores in which they live; the Polycystids lose the epimerite wholly or in part, e.g. in Clepsidrina, or its retinacular processes are alone absorbed (Echinocephalus). A conjugation so-called may take place; the two anterior extremities come into contact (apposition), as in Monocystis, etc.; or the anterior extremity of one individual with the posterior of another (opposition), as in Polycystids, leading occasionally to the formation of strings of individuals. The union is more or less firm 1 Solitary individuals may encyst, or conjugated individuals may become inclosed in a common cyst. They may, as is probably generally the case, be separated by a septum, or they may rotate over one another and perhaps in some instances fuse. Encystation is always accompanied by the assumption of a globular shape. The cyst is said in Monocystis to be formed by the cuticle and sarcocyte; in the majority it is certainly a new structure. It may be single or double, and the outer layer sometimes thick and concentrically striate. A delicate inner membrane is present when there are special sporoducts (Gamocystis, Clepsidrina). The outer membrane is ornamented with tubercles, etc, in Stylorhynchns and its allies.

The size of the cyst varies; it is resistent to the action of water. It undergoes fission in Porospora gigantea; the two first formed halves divide a second time, and the old cyst-wall degenerates into a transparent mass imbedding the cysts secreted by the new parts.

1A parasite, Drepanidium Ranarum, inhabits the red blood-corpuscles of the Frog, and has a close structural resemblance to a falciform body, and is perhaps derived from a Coccidium infesting the intestinal and renal epithelia. It has been observed to quit one corpuscle and enter another. Other Gregariniform parasites have been found in the blood of Emys lutaria, Lacerta viridis, and several birds. See Ray Lankester, Q. J. M. xxii. 1882; Danilewski, A. M. A. xxiv. 1885; Id. Biol. Centrablatt. v. 1885-6, and Archives Slaves de Biologie, i. 1886.

The contents of the cyst proceed to develope either while it is in situ, or after it is expelled in the faeces, if the Gregarine inhabits the digestive tract. The nucleus becomes invisible; many superficial nuclei, doubtless derived, as in Coccidiidae, by fission of the primitive nucleus, have been observed in Clepsidrina Blattarum. The protoplasm either segments entirely into a number of nucleated sporoblasts, or only the superficial zone does so. In the Monocystids of the Earthworm, the sporoblasts are said to be formed centrally, to be extruded successively, and arranged in concentric zones, until the whole protoplasm is used up. They are formed as buds projecting from the surface in Clepsidrina and Stylorhynchus, in the last-named motile. The protoplasm not used up is usually resolved into a fluid; in Stylorhynchus it collects into a central spherical mass, the pseudocyst. The sporoblast assumes by degrees its definitive shape, elliptical and pointed at the ends, which in Syncystis have four bristles, cylindrical with conical truncate ends, as in Clepsidrina, or purse-shaped, as in Stylorhynchus and its allies.

In Urospora it has a tail2. It acquires an envelope, single or double, as in Clepsidrina, bivalved in Adelea, porous in Porospora, where it is readily resolved into minute rods; brown or black in Stylorhynchus and its congeners. Spores belonging to the same species may be large in one cyst, small in another (macro-, micro-spores). Except in Porospora gigantea the contents are resolved into falciform, bodies or sporozoites, a small quantity of protoplasm remaining as the 'nucleus de reliquat' The number of falciform bodies in each spore-case is probably a constant one; eight, as in Clepsidrina, appears to be very general. The falciform body is homogeneous, pointed at one end, rounded at the other, or terminated by a fine process, as in Stylorkynchus, and it has been found to be nucleated whenever properly examined1.

1The genus Didymophydes of Stein was founded on two conjugated Sporonts. Frenzel has established a genus Aggregata for a Polycystid inhabiting the intestine of Cancer Maenas, which, conjugates in strings of individuals.

2The elliptical spore pointed at both ends is sometimes termed 'pseudo-navicella.' The term 'psorosperm' has been applied to the Coccidiidae, Gregarine spores, Myxosporidian spores, and to the Sarcosporidia.

The spores are discharged from the cyst by special tubular sporoducts in Clepsidrina and Gamocystis; in other instances by its disruption caused by an accumulation of liquid (?), or in Stylorkynchus by the swelling of the pseudocyst. They are set free either perfectly loose or else cohering end to end in chaplets. The escape of the falciform bodies from the spore-case by its dehiscence along one edge has been observed in Stylorkynchus longicollis infesting Blaps when the case was exposed to the action of the digestive fluids of the host. The bodies in question executed movements by bending from side to side. The presence of a small nucleated oval or rounded cell, formed beyond doubt from a falciform body, in the intestinal epithelium of the proper host, the growth from it of a process which gradually projects into the cavity of the digestive tract, the appearance of constrictions dividing the growth into segments (supra), and the descent of the nucleus into the deuteromerite before the completion of the septa, have been recorded in several Polycistids. The spore of Porospora gigantea gives origin, according to E. Van Beneden, to a non-nucleated amoeboid 'moner'; this in its turn to two processes, one of which grows faster or starts earlier than the other.

The longer process is vibratile and breaks away; the other becomes vibratile as soon as it has absorbed the remaining protoplasm. Each process is termed by Van Beneden a 'pseudofilaria'; it turns motionless after a time, developes a nucleus, lengthens, becomes septate and adult. Urocystis (= Monocystis) Sipunculi is said by Lankester to have a 'pseudofilaria' stage, followed by a 'pseudocercaria' stage, i. e. one with a slender tail and large body like a Cercaria. The body is nucleated, the tail drops away, and its fate is unknown. It must be borne in mind that the life-history of only a few Gregarinida has been traced.

Longitudinal fission occurs in the young Urocystis Sipunculi; transverse fission into two or three parts in the adult Monocystis porrecta and M. cuneiformis of the Earthworm, the parts thus formed undergoing encystation at the same time.

The Coccidiidae are found in various Vertebrata, Mollusca and In-secta; the Monocystidae chiefly in Urochorda, some Chaetopoda, Gephyrea, Nemertea and Turbellaria, rarely in Arthropoda; the Polycystidae, the division to which the greater number of genera belong, are almost confined to Insecta, but have been found also in Phalangidae and some A carina among Arachnida, and two are known from Urochorda.

1Ruschhaupt maintains that in the Monocystis of the Earthworm the falciform body is a mass of reserve material; that the rest of the protoplasm grows at its expense. The ripe spore set free by the rupture of the cyst is taken up by an amoeboid spermatospore of the vesicula seminalis. The spore-case slowly thins away, and by growth the spore passes into the adult. Cf. Schneider's critique in his Tablettes Zoologiques, i. 1886.