II 



PHYLUM PROTOZOA 



59 



Many genera form colonies. Numerous zooids may be united 

 by bridges of protoplasm into an open network, or the connecting 

 bridges may be shorter and the zooids more numerous, giving the 

 colony a more compact appearance. 



Transitional stages occur between the naked genera already re- 

 ferred to and forms with a distinct skeleton. Sometimes the body 

 simply surrounds itself with a temporary gelatinous investment 

 (Fig. 41, 2,g.), in other cases it is surrounded by a capsule of loosely 

 woven fibres through which the pseudopods pass, thus reminding 

 us of the state of things characteristic of perforate Foraminifera. 



Fio. 42.— Actinophrys sol. Conjugation with fusion of nuclei (karyogamy). A. two indi- 

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

 f), 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 ; 3, nucleus ; 

 3, spindles concerned in maturation ; U, 5, outer and inner layers of cyst ; 6, polar bodies ; 

 7, nucleus formed by the union of the two nuclei; S, first (mitotic) division. (From Lang, 

 after Schaudinn.) 



One genus has a shell formed of agglutinated sand-grains; in 

 another (Fig. 41, 1) the skeleton consists of loosely matted needles 

 of silica. Lastly, in the graceful Clathrulina (3) the body is 

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

 many of the Radiolaria (p. 61). 



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 



