168 THE PROTOZOA 



Another example of a parasite with alternation of hosts, in which 

 the course of events is different from that of the malarial parasites, 

 is furnished by the species of the genus Aggregate (p. 353). Here 

 the schizonts are parasitic in crabs, and reproduce themselves by 

 multiple fission without encystment to form naked germs, mero- 

 zoites, which grow up into schizonts, and multiply again in the same 

 way. If, however, the crab is devoured by a Cephalopod, the 

 merozoites adapt themselves to their new surroundings and become 

 sporonts, which produce gametes. The zygotes form resistant 

 cysts in which they multiply to form spores enclosed in tough 

 sporocysts. The resistant phases pass out of the Cephalopod in its 

 faeces, and to develop further they must be devoured by a crab, in 

 which they become schizonts again. In this case there is no special 

 differentiation of propagative phases in the crab, but the same 

 stage can serve both functions ; on the other hand, in the 

 Cephalopod there is no multiplicative phase, but only a propagative 

 phase with resistant cysts. 



2. Polymorphism in Relation to Growth and Development of the 

 Individual. In Protozoa which multiply only by equal binary 

 fission, as, for example, many Infusoria, there is practically no 

 difference between young and old forms beyond a slight variation 

 in size. An individual feeds, and in consequence grows slightly 

 beyond the size characteristic of the species to which it belongs. 

 It then divides by equal binary fission into two individuals each 

 slightly below the specific size, and they in their turn feed and grow 

 and reproduce themselves by fission in due course. 



In other cases, however, young and adult forms of a species can 

 be clearly distinguished, and may differ in structure as well as in 

 size. Beginning with reproduction by binary fission, the simplest 

 case is where the adult individual divides into two unequal portions, 

 so that parent and daughter can be distinguished, the former not 

 appreciably smaller than ordinary full - grown individuals, the 

 latter, however, very much smaller ; it may be relatively minute. 

 Examples of this type of reproduction are furnished by trypano- 

 somes, a group in which all gradations may be found between equal 

 and very unequal fission (Fig. 127). Still greater differences 

 between parent and young individuals are seen in cases of gemma- 

 tion that is to say, where the offspring is set free in an undifferen- 

 tiated condition, and acquires after separation from the parent the 

 characters of the adult, as in Acinetaria. 



The greatest differences between young and old forms are seen, 

 as might have been expected, in cases of reproduction by multiple 

 fission or gemmation. In such cases the young forms produced 

 often differ from the adult in structure and appearance, as well as 

 in size. An example of multiple fission is furnished by the common 



