336 Ciliophora 



Binary fission is unusual in that the plane of division is oblique, or 

 almost longitudinal, instead of transverse as in typical ciliates. In the 

 binucleate Zelleriella elliptica (38), fission often precedes the completion 

 of mitosis and produces temporarily uninucleate daughter organisms. 

 Although nuclear behavior is not easily traced in multinucleate species, 

 series of nuclear divisions apparently may precede and follow fission. 

 Behavior of the ciliature and fibrillar system in fission of Opalina ra- 

 narum has been described (21). 



Little is known about the life-cycles of Protociliatia. However, the 

 cycle of Opalina ranariim, according to one account (123), is fairly com- 

 plicated. Throughout most of the year large multinucleate forms are 

 present in the host. In the spring, plasmotomy produces small ciliates 

 containing only a few nuclei. These stages encyst and are eliminated by 

 the host. Such cysts are ingested by tadpoles and liberate "gametocytes." 

 Repeated division of the "gametocytes" results in "gametes" of two sizes. 

 According to another version (160), some ingested cysts give rise to 

 gametes while others hatch into ciliates which merely grow to maturity. 

 Gametes similar to those reported for O. rananun have been described in 

 Protoopalina intestinalis (151). Gametogenesis is said to be followed by 

 anisogamy. The resulting zygotes encyst and are eliminated from the 

 intestine. After ingestion by another tadpole, each zygote excysts and 

 develops into a multinucleate ciliate. Encysted adults, as well as small 

 cysts containing 1-4 nuclei, have been observed in Opalina chattoni (217). 



Unfortunately, the behavior of chromosomes has not yet been traced 

 throughout the life-cycle and meiosis remains to be described. However, 

 a few apparently haploid specimens have been noted in Zelleriella louisi- 

 anensis (38). These ciliates were approximately normal in size but con- 

 tained four relatively small nuclei, each with 12 chromosomes, instead of 

 the usual two nuclei with 24 chromosomes each. 



Taxonomy 



Flagellate affinities have been suggested for the Protociliatia (73, 118). 

 On the other hand, nuclear structure and mitosis in Opalina ranarum 

 and O. obtrigona show features characteristic of ciliates (92) and the 

 fibrillar system of O. ohtrigonoides resembles that of holotrichous ciliates 

 (43). Accordingly, retention of the opalinid ciliates in the Ciliatea appears 

 to be sound practice at present. The evident lack of macronuclei is a 

 logical basis for continued recognition of Metcalf's Subclass Protociliatia. 



The most extensive taxonomic work on the group is that of Metcalf 

 (152, 153), who established two families on the basis of nuclear number. 

 The Protoopalinidae include binucleate types assigned to two genera: 

 Protoopalina Metcalf (8, 152; Fig. 7. 1, F, I), with cylindrical or slightly 

 flattened bodies; and Zelleriella Metcalf (7, 37, 52; Fig. 7. 1, G), with dis- 

 tinctly flattened bodies. The multinucleate types are placed in the Opa- 



