24 MORPHOLOGY OF PROTOZOA 



gregarines the uniting cells difier from one another (anisogamy), so that 

 there is a degree of polymorphism as regards the character of the gametes 

 (Fig. 482). It may happen that the individual which gives rise to gametes 

 of one type differs from that which gives rise to gametes of the other type. 

 This differentiation may extend further back in the life-history, so that 

 it is possible to recognize two distinct types of reproducing individual, 

 each with its particular characters (Fig. 341). The individuals of one 

 series may eventually, after a period of multiplication, give rise to gametes 

 of one type, while those of the other series give rise to gametes of another 

 type. In such cases it might be supposed that one was dealing with two 

 distinct organisms, each reproducing its kind. The fact that the gametes 

 produced by the one unite with those produced by the other proves that 

 the two series belong to one polymorphic species. This condition is known 

 as one of sexual dimorphistn, a term which is also employed in a more 

 general sense to indicate the occurrence of individuals of any species 

 which can be distinguished as male and female. 



Though all these variations in form, which occur as a result of growth, 

 complicated life-cycles or the sexual process, are examples of polymorphism, 

 the term is often employed in a more restricted sense. When it has been 

 decided which stage of the organism is to be regarded as the adult form, 

 it may be found that the adults resemble one another very closeh^, in which 

 case the organism is said to be monomorphic. Thus, in the case of 

 trypanosomes the commonly observed forms in the blood of an animal 

 may vary very slightly. In these cases the organism is termed a mono- 

 morphic trypanosome, examples of which are Trypanosoyna evansi and 

 T. congolense (Figs. 227 and 234). In other cases, as, for instance, Try- 

 panosoma brucei, it may be possible to distinguish in the blood of an animal 

 several distinct types — long thin, intermediate, and stumpy trypanosomes 

 — and forms with or without free flagella (Fig. 225). On this account 

 T. brucei is regarded as a polymorphic trypanosome. If, however, the 

 whole life-cycle in the vertebrate and invertebrate hosts of such a form as 

 T. lewisi, which at certain phases appears monomorphic, is taken into 

 consideration, it will be found to exhibit as great a degree of variation as 

 in the polymorphic trypanosomes (Fig. 197). 



It seems clear, therefore, that the term " polymorphism " is incapable of 

 exact definition. Strictly speaking, no Protozoan is monomorphic, while 

 all are polymorphic. Those which are considered monomorphic show 

 only a slight degree of variation, while those which are polymorphic show 

 the variations, but to a greater extent. Any organism may be regarded 

 as polymorphic because it differs at different stages of its growth and life- 

 history, or it may be considered as polymorphic because the individuals 

 which have all reached any particular stage do not resemble one another 



