This, roughly, is syzygy, a key event in the life cycle of many of the invertebrate-gut-parasitising protists known as eugregarines. Originally, the term 'syzygy' referred to the conjunction of two heavenly bodies, and provides a very poetic label for the process by which two of these unicellular organisms conjoin, rotating around one another as they produce and outer membrane to contain themselves within a single gametocyst. Once within the gametocyst, each divides into numerous gametes (which are produced through straight mitotic division, as eugregarines are haploid at maturity rather than diploid like ourselves). The resulting gametes will then be released from the gametocyst to fuse with one another in the production of diploid zygotes. Each zygote encloses itself in a resistant oocyst, in which state it may be passed out of the host's digestive system and be swallowed by a new host. While within the oocyst, the zygote divides to produce a number of new haploid individuals. Once the oocyst is in a suitable host, the new eugregarines are released, ready to feed and hopefully to eventually find a syzygy of their own.
Eugregarines have been referred to at this site before. As described in that post, they are part of the group of protists known as gregarines. Eugregarines differ from the other two major subgroups of gregarines, the archigregarines and neogregarines, in that they do not include an extensive asexually reproducing phase in their life cycle in addition to the sexual phase. All known eugregarines are parasites of invertebrates: their hosts include arthropods, molluscs, annelids and tunicates. Most eugregarines parasitise only a single host species over the course of their life cycle. The only known exception is members of the family Porosporidae, which are believed to spend part of their life cycle in a crustacean, and part in a mollusc. However, the porosporid life cycle has only been observed in its entirety once in 1940, when H. F. Prytherch fed infective spores from an oyster to crabs. It has been suggested that Prytherch may have conflated two separate parasites, with the eugregarine infection observed in the crabs after feeding them the spores actually representing a pre-existing infection that they had been carrying before the start of the experiment (Clopton 2002).
The eugregarines are usually divided between three suborders. Two of these, the Septatorina and Aseptatorina, include the great majority of species and are distinguished (as their names suggest) by the presence or absence of septae dividing the cell into sections. The third suborder includes the single small genus Siedlickia, parasites of marine annelids, which differs from other eugregarines in that it does not go through syzygy; instead, reproductive cells are budded directly off the mature feeding cells. The relationships between the three suborders are largely unknown; the Aseptatorina in particular seems to be defined largely by plesiomorphies. Clopton (2009) argued for a marine ancestry of eugregarines as a whole, and that the radiation of the septate eugregarines had been driven by adaptations of the gametocyst allowing their transmission in freshwater and terrestrial habitats. However, both the aseptate and septate eugregarines include parasites of marine, freshwater and terrestrial hosts. The fact that Clopton did not refer in 2009 to the marine members of the Septatorina (in the Porosporidae and various families of the Gregarinoidea) is somewhat bemusing as he himself had reviewed them some years earlier in his 2002 chapter on the eugregarines for The Illustrated Guide to the Protozoa. It is possible that he simply assumed the marine species to sit outside the terrestrial-freshwater clade, but it would have been nice for hime to say so.
Ignorance of the marine eugregarines does seem to be a theme, though: they're definitely less well-studied than the parasites of terrestrial species. Not that the latter can claim to have been exhaustively studied either: as noted by Clopton (2002), while over 1600 species of eugregarine have been described, only a fraction (much less than one percent) of potential hosts have been investigated for their presence. As almost every investigation of a new host results in the description of new parasite species, it is possible that the total number of eugregarine species out there ranks in the millions. Eugregarines are morphologically and behaviourally diverse. Attachment to the cells of the host's intestinal lining is via a structure called the epimerite, which may be a simple nubbin or may be a complex branching, fingered, collared or dart-like structure. When not attached to the host cell, most eugregarines move by gliding, but the worm-like Selenidiidae move by nondirectional swinging or thrashing. Many taxa are all distinguished by the characteristics of their syzygy. They may connect end to end, or they may lie top-to-tail. Members of the septate superfamily Gregarinoidea form associations some time before entering actual syzygy, so they are often found connected (whereas other taxa that do not become conjoined until the point of syzygy are more often found as isolated cells). Syzygy is most often between two individuals, but some Gregarinoidea regularly form associations of three or more. At least one species, Hirmocystis polymorpha, has been found in head-to-tail chains of up to twelve individuals. Whether such polygamous associations lead to all the individuals involved combining to form one gametocyst, or whether some form of competition occurs to whittle them down to a single victorious pair, is something I haven't yet discovered.
Clopton, R. E. 2002. Order Eugregarinorida Léger, 1900. In: Lee, J. J., G. Leedale, D. Patterson & P. C. Bradbury (eds) Illustrated Guide to the Protozoa, 2nd ed., vol. 1 pp. 205–288. Society of Protozoologists: Lawrence (Kansas).
Clopton, R. E. 2009. Phylogenetic relationships, evolution, and systematic revision of the septate gregarines (Apicomplexa: Eugregarinorida: Septatorina). Comp. Parasitol. 76 (2): 167–190.