1. A study of the bionomics of L. tattersalli shows that the initial infection of the host is by the passage of swarmers from the gill-plates of the female host to those of the embryo in the brood-pouch. The transmission of the infection from instar to instar is about 50-70 per cent, effective. It is brought about by (i) the mobilization of the entire colony at ecdysis by an obligatory type of division (second-type division), and (ii) by the penetration of the swarmers to the surface of the new instar through splits which appear in the old cuticle as it is shed.

2. The term ‘protoconjugant’ is applied to the organism from which the two microconjugants are formed by binary fission. The protoconjugant is formed by the unequal fission of the parent, and not by budding as in L. platei (Ubisch, 1913). The divisions which produce the proto- and microconjugants are compared with the normal asexual reproductive process (first-type division, Willis, 1942).

3. The conjugation process is described. In view of the early fusion of both conjugants, the term ‘synconjugant’ is proposed for the macroconjugant after its fusion with the microconjugant. There are no visible nuclear preparations for conjugation in either conjugant before their attachment. The evidence for a state of ‘Conjugationserwartung’ (Ubisch, 1913) is criticized and shown to be based, in all probability, on specimens produced by the distributive divisions of the synconjugant.

4. The term ‘necrochromidium’ is proposed for the mass of vesicles formed by the degeneration of the macronucleus in conjugation and endomixis. Before the formation of the necrochromidium the macronucleus of the macroconjugant becomes elaborately branched.

5. The distributive divisions of the synconjugant are described for the first time. Resemblance to other vorticellinids is shown during the intermoult period of the host. At ecdysis, the distribution of the nuclear Anlagen is adapted to the obligatory second-type divisions which are undergone by L. tattersalli at this period. In some cases one of the divisions of the synkaryon is cytoclastic.

6. A process of apparent endomixis is described. In this process unequal fission produces a small organism, resembling a protoconjugant which fails to separate, and a larger organism. The micronucleus of the smaller organism remains undivided and soon degenerates. The micronucleus of the larger organism undergoes two successive divisions to form four nuclear Anlagen.

7. An anomalous process of micronuclear division and reduction is described. One of the micronuclei produced by the division passes into a small bud of protoplasm. Later the latter falls off and degenerates.

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