There is a process known as conjugation which is generally if not universally connected with reproductive activity in some Protozoan classes (Acinetaria, Infusoria, many Mastigophord); whilst in others it is either of fortuitous occurrence, e. g. in Gregarinida, or it does not so far as is known occur at all, e. g. in Radio-laria, Foraminifera, It consists of a temporary or permanent union of two or rarely more individuals. When it is permanent, the two individuals may be similar so far as external signs are concerned, but they are sometimes totally unlike and with a different life-history, as in the Infusorian Vorticellidae and many Flagellata. When it is temporary, the two are alike, and during its progress they may or may not undergo as in some Infusoria a loss of their locomotor organs. In either case, one result attained is a mingling of two different protoplasms. When temporary, an interchange of nuclear bodies has been observed (note 2, p. 835); and whether temporary or permanent, disruption and reconstruction of the nucleus may take place.

If temporary, the process is followed by growth (rejuvenescence) and growth in turn by fission: so too if permanent in some instances, with or without an intervening period of quiescence, but in others the fusion is followed by a formation of many spores, e.g. some Flagellata. There can be no doubt that the process is essentially a sexual one and that it inaugurates a new departure in the history of the individual. Indeed when the conjugating individuals are invariably different there is no reason why the terms male and female should not be applied to them. If conjugation fails to take place, where it is of normal occurrence, it has been noticed that the race becomes extinct, e. g. in some saprophytic Flagellata. The rapid decrease of size brought about by repeated fission in Infusoria is always followed by an epidemic of conjugation. The absence and origin of the process are alike difficult to explain. It is of course impossible to say that it never occurs in any given case: it has simply not been observed. As to origin, it is possible but not likely that it has come from associations of individuals united apparently for the better procuring of food, as is seen in some Rhizopoda, e. g.

Actinophrys, Microgromia. Such unions do not appear to take place in Protozoa with a dominant ciliated or flagellate phase. For a recent theoretical discussion of the significance of the act, see Plate, Z. W. Z. xliii. p. 215 et seqq., p. 2391.

Encystation is a process universal among Protozoa. It is invariably protective, but the protection subserves different ends, either against unfavourable conditions of life (the hypnocyst as it is termed), for the purposes of digestion or reproduction, or for a necessary period of quiescence. The appearance and structure of the cyst may vary in accordance with the object to be attained in the same individual. It may be single, double or multiple; simple or ornamented with spines, etc.; colourless or coloured, and very frequently brown; in substance gelatinoid, chitinoid, of cellulose, or a cellulose-like material.

The Protozoa are aquatic, and inhabit both fresh and salt water; some are parasitic either ecto- or endo-parasitic; some are capable of a sub-aerial life in moist places. Some are social, others colonial, i.e. connected by processes which are integral parts of the body. Many of the freshwater forms are subject to the attacks of fungal parasites (Chytrideae) or Bacteria, which have been the cause of erroneous views as to their reproduction. Some of the marine forms, e. g. Radiolaria, are inhabited by symbiotic yellow algae, and in one or two instances by symbiotic diatoms. A few Amoebae appear to harbour symbiotic (?) fungi.

1Gruber's paper, referred to by Plate, is translated in A. N. H. (5) xvii. 1886.

The classes of Protozoa are for the most part sharply delimited. The position of a few genera is indeterminate, and one group, Proteomyxa, contains organisms which at present cannot find a place elsewhere. Doubts attach to the animal nature of some, e. g. the Flagellate Volvocina. The question, however, as to what constitutes an animal is treated in the General Introduction.

A subject which presents some difficulty is the mode in which the classes are to be grouped. Biitschli has not yet published a general introduction to his 'Protozoa '; and the only writer who has dealt with this division of the Animal Kingdom in its entirety, from a modern point of view, is Ray Lankester in his article 'Protozoa' cited p. 824. By him the classes are grouped in two main divisions, Gymnomyxa and Corticata. The essential features of the first-named are (1) that the protoplasm of the vegetative phase of life is naked, i. e. partially or wholly exposed to the surrounding medium; (2) that solid food may be ingested at any spot or at any spot of a large limited area; (3) that the distinction between exo- and endoplasm sometimes recognised is not a permanent one, exoplasm becoming endo-plasm and vice versa at different times. The Corticata on the contrary have the protoplasm of the body permanently differentiated into two layers, an outer denser cortical substance and an inner more central fluid substance.

In this division are included the Acinetaria, Infusoria, Mastigophora, and Sporozoa.

Certain objections present themselves to this mode of grouping which may be briefly summarised. Whilst it is perfectly certain that in some Gynmomyxa, e. g. Foraminifera, some very fluid Amoebae, the structure of the body is entirely similar throughout, it is by no means so certain that a constant intermixture of the peripheral and central protoplasm is always taking place in some of the more differentiated Amoebae and more especially most Heliozoa. Nor on the other hand is a differentiation into a permanently distinct- cortex and medulla to be found in many Corticata: e. g. the majority of Flagellata, some Infusoria, and Acinetaria. It is possible indeed that the degree of distinctness may vary with the state of nutrition. Some Flagellata lose their permanent outline and become amoeboid; some Gymnomyxa, like the Heliozoa, always have a permanent outline, and when they pass into a flagellula, e. g. the fission products of Clathrulina or the spore of Myce-tozoa, the outline of the body is invariably definite. Indeed certain adult forms, denominated by Butschli Rhizomastigina (p. 841), may pass at will from one condition to another.

If the pseudopodia of Gymnomyxa are extensions of the protoplasm, so are also cilia, flagella, membranellae, and they may even be retracted under certain conditions. There is also no absolute distinction as mentioned above (p. 819) between the two kinds of processes, and sometimes, e.g. in the Heliozoa and some Radiolaria, the pseudopodia are remarkably stable structures. Furthermore what is to be said as to the skeletal structures of both divisions ? Gradations appear to exist in Infusoria, for example from a naked condition to one in which there is a somewhat modified superficial stratum of protoplasm, and from this in turn to a close-fitting or detached cuticular structure. Similar gradations may be traced in Amoebina; and it may be pointed out that nothing can exceed the definiteness of outline conferred by the skeletal structures of many Gymno-myxans.

In the following pages the Protozoa are thrown into three divisions, depending on their modes of locomotion and of obtaining solid food, as well as on the dominant or adult phase of the life-history. For the Gymnomyxa the old term Rhizopoda is retained. The Sporozoa, which must be regarded as Rhizopods adapted to an endoparasitic life, are kept apart as Endoparasita. For the Acinetaria, Infusoria and Mastigophora, the designation Plegepoda is proposed, referring to their mode of progression by means of a rapidly repeated stroke (nXrjyrj) of vibratile processes.

For Erythropsis agilis, a Protozoon (?) of very peculiar structure, with an eye composed of a lens and pigment, known only from a single specimen, see R. Hert-wig, M. J. x. 1885; C. Vogt., R. Hertwig, Z. A. viii. 1885. Metschnikoff (Z. A. viii) states that he once found a very similar organism which he believes to have been an Acinetarian.

For Dumontia Opheliarum, an endoparasite in the coelome of the Chaetopod Ophelia, which has a peculiar internal axis, a bilobed body with stable pseudopodia, and vacuolate exoplasm, and a peculiar mode of gemmation, see Kunstler, Bull. Soc. Zool. France, x. 1885. Kunstler is of an opinion that it is the type of a new group of Rhizopoda.

'Protozoa,' Biitschli, Bronn's Klass. und Ordn. des Thierreichs, Leipzig, 1880-5; does not yet include the Acinetaria and Infusoria. 'Protozoa,' Ray Lan-kester, Encyclopaedia Britannica (ed. ix), xix. 1885.