II 



PHYLUM PROTOZOA 



61 



enclosed in a perforated sphere of silica, quite like the skeleton of 

 many of the Radiolaria (p. 62). 



Reproduction ordinarily takes place by binary fission : a 

 peculiar form of budding has been observed, and spore-formation 

 also occurs, with or without encystation. Actinosphaerium, for 

 instance, encloses itself in a gelatinous cyst and undergoes 

 multiple fission, forming numerous spores each enclosed in a 

 siliceous cell-wall. These resting spores remain quiescent 

 throughout the winter, and in spring the protoplasm emerges 

 from each and assumes the form of the ordinary active Actino- 

 sphserium. In Clathrulina spore-formation takes place in the 



A\ 



1-4-3 





-\ 6 



\7 



Fia. 43. Actinophrys sol. Conjugation with fusion of nuclei (karyogamy). A, two indi- 

 viduals in the first phase (^conjugation ; B, beginning of the encystation ; C, maturation; 

 Z>, completion of maturation ; E, coalescence of nuclei ; F, completion of the first spindle 

 of the zygote resulting from the conjugation. 1, axial filaments of the pseudopods ; 

 2, nucleus ; 3, spindles concerned in maturation ; 4, 5, outer and inner layers of cyst ; 

 6, polar bodies ; 7, nucleus formed by the union of the two nuclei ; 8, first (mitotic) 

 division. (From Lang, after Schaudinn.) 



active condition, and the spores (Fig. 42, 3 b] are flagellulse, each 

 being an ovoid body provided with two flagella. 



Conjugation 1 has been observed in some instances between two 

 or more individuals, which may separate again without any nuclear 

 changes taking place ; or the conjugation may be followed by a 

 process of copulation, comprising the coalescence of the protoplasm 

 of the two individuals and the coalescence of the nuclei (Fig. 43) 

 after each has given off a part of its substance (6), as in the 

 maturation of an ovum in multicellular animals (p. 20). Autogamy 

 (p. 51) also occurs in both Actinophrys and Actinosphserium. 



1 The term conjugation is here restricted to an association involving close 

 approximation without complete coalescence (copulation). 



