170 THE PROTOZOA 



and become a flagellula ; after a time the flagellula settles down and 

 becomes an amcebula again after loss of the flagellum. 



A very interesting point, in connection with the question of young 

 and adult forms of Protozoa, is the occurrence of stages in the 

 development which may be interpreted as recapitulative in the 

 phylogenetic sense that is to say, as representing past stages in the 

 evolution of the species, in a manner comparable to the recapitu- 

 lative larval or embryonic stages in the development of Metazoa. 

 It is probable that such recapitulative stages are commoner in the 

 development of Protozoa than has been generally supposed (compare 

 Awerinzew, 47). The best-known instance is furnished by the 

 ciliated larvae of Acinetaria (p. 459), indicating that this order is 

 descended from a ciliate ancestor of the order Peritricha, a relation- 

 ship fully confirmed by the similarity of their reproductive processes 

 to those of other Infusoria. The crithidial phase that occurs so 

 constantly in the development of trypanosomes (p. 299) is probably 

 to be regarded as a recapitulative form representing a type of 

 structure antecedent in evolution to that of the typical trypanosome- 

 form. The frequent occurrence of flagellated swarm-spores in the 

 development of Sarcodina (Foraminifera, p. 235 ; Radiolaria, p. 254) 

 probably has a phylogenetic significance, as pointed out by Biitschli. 

 Finally attention may be drawn to the remarkable series of forms 

 in the ontogeny of Arcella described in the next chapter ; first the 

 amcebula, then the Nuclearia-st&ge, followed by the Pseudochlamys- 

 stage, which grows finally into the adult Arcella-iorm. In the many 

 cases where young forms are markedly different from the adult, it 

 may be a difficult matter, as it often is in the case of Metazoa, to 

 decide whether a given larval form is to be interpreted as recapitu- 

 lative or merely adaptive ; but even in cases where the characters 

 of a larval form have an obvious adaptive importance, as in the 

 ciliated larvae of Acinetaria, atavism may be nevertheless a factor 

 determining the particular form taken by the adaptive characters in 

 question that is to say, by the organs of locomotion in the example 

 chosen. 



3. Polymorphism in Relation to Sex. The phenomena of sexual 

 differentiation consist primarily of differences in size, structure, and 

 other characteristics between the gametes, the cells which are con- 

 cerned in the act of syngamy. Secondarily such differences may 

 extend to other cell-individuals, both in the life-cycle of a Protozoon 

 or in the body of a Metazoon. In the previous chapter it has "been 

 pointed out that, while in Metazoa the gametes at least are sharply 

 differentiated in all cases, in the Protozoa every condition is found 

 from perfect isogamy to a differentiation nearly as pronounced as 

 that in the Metazoa. The question has been discussed in the last 

 chapter whether or no sexual differentiation is to be regarded as 



