24 THE INVERTEBRATA 



The fission of the Protozoa takes place in several ways. Whether in 

 asexual reproduction or in the formation of gametes, it may be: 

 (a) equal binary fissioji, the familiar mode of division of Afnoeba, 

 Para?necium, and a vast number of other cases; (b) budding, in which 

 one or more small products separate from a parent body, as in 

 Arcella (Fig. 22 B), the Suctoria (Fig. 95), etc.; {c) repeated fission, 

 in which equal divisions give rise to four or more young which do not 

 separate till the process is completed, as in Chlamydomonas (Fig. 23), 

 the microgamete formation of Volvox (Fig. 44), etc.; or {d) multiple 

 fission, in which the nucleus divides several times without division 

 of the cytoplasm, which finally falls into as many parts as there are 

 nuclei, usually leaving behind some residual protoplasm, which may 

 contain nuclear matter. Multiple fission is seen in the spore formation 

 of numerous protozoa, as Amoeba, sporozoa (Fig. 74 C, D; Gg, G3; 

 K, L), etc. The fission of multinucleate protozoa, such as Actino- 

 sphaerium, Opalina (Fig. 85), etc., to form multinucleate offspring by 

 division of the cytoplasm without relation to that of the nuclei, is 

 known as plasmotomy. It is usually binary, but occasionally takes 

 place by budding or is multiple. The plane of simple binary or of 

 repeated fission is often transverse to the principal axis — if there be 

 one — of the body, but in most flagellates it is longitudinal. Repeated 

 longitudinal fission in which the daughter individuals remain in 

 position is called radial ; such fission is common in the green flagellates 

 of the order Volvocina (e.g. some species of Chlamydomonas, Fig. 23 

 A-D). Sometimes an individual in longitudinal fission shifts in its 

 cuticle during the process, till the plane of division becomes transverse. 

 Fission of this kind is said to be pseudotransverse: it is seen, for 

 instance, in some Chlamydomonas (Fig. 23 E-H). In Polytoma (Fig. 24) 

 the only vestige of longitudinal fission consists in a slight obliquity of 

 the first division of the nucleus. 



Each type of fission takes place in some cases in a cyst and in other.s 

 without encystment. 



The fate of flagella at fission varies. Sometimes, as in Chlamy- 

 domonas and Polytoma, they are lost, early or late in the process. In 

 other cases they are retained. When this happens in an organism 

 with a single flagellum, that organ has been said sometimes to be split 

 longitudinally, but usuaUy, if not always, a second flagellum grows 

 out from the basal granule, which divides. When several flagella are 

 present and persist, they are distributed between the offspring, each 

 of which grows new flagella to complete its equipment. Probably, 

 a new flagellum always grows from a basal granule. Chromatophores 

 divide, and if numerous may do so independently of the fission of the 

 body. Contractile vacuoles and other organs rarely (Euglena) divide, 

 but are usually shared as the flagella. Complex organs, however, are 



