822 THE ANIMAL KINGDOM. 



to which this point has given rise, see Btitschli, Cholodkowski and Weismann, Z. A. 

 v. 1882 : Gotte, 'Uber den Ursprung des Todes,' Hamburg, 1883; Weismann, 

 'Uber Leben und Tod,' Jena, 1884; Mobius, 'Das Sterben,' etc., Biol. Central- 

 blatt. iv. p. 389. 



There is a process known as conjugation which is generally if not universally 

 connected with reproductive activity in some Protozoan classes (Acinetaria, Infu- 

 soria, many Mastigophora) 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- 

 taria, 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 tem- 

 porary 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 etseqq., p. 239 '. 



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 



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



