OPALINID CILIATE INFUSORIANS — METCALF 559 



But the variation in form is more than a matter of a flexible body 

 assuming temporarily different shapes. Tliis may account for only 

 a small part of the variation. At the posterior end of the body therp 

 may be, for example, a slender, tapering, naked "tail," like a whip- 

 lash. In some species this seems to be always seen (fig. 21, Protoopa- 

 lina, subgenus I) (cf. also figs. 12, 13, and 36 in Metcalf, 1923a). In 

 other species it appears in some individuals but not in others. Com- 

 pare P. caudata, in which, in the same individual host may sometimes 

 be seen individuals with slender tails and others with rounded poste- 

 rior ends (see fig. 19 in Metcalf, 1923a). Some infections of P. caudata 

 may contain only posteriorly rounded individuals; other infections 

 contain mostly tailed forms with some rounded ones; still others may 

 show mostly rounded ones and a few with short, curved, posterior 

 hook, either sharp or with a blunt point. These are not temporary 

 conditions assumed by any individual at will, but are constant and 

 characteristic for the individual. Furthermore, there are found, in 

 addition to the comparatively slender individuals, others that are 

 very broad and swollen (see fig. 17, a, in Metcalf, 1923a). One does 

 not know how to regard the great divergence found. Is it an indi- 

 cation of races within the species? It may be. Probably only breed- 

 ing experiments would solve the puzzles, and the peculiar parasitic 

 habits make such experiments forbiddingly difficult. Another variety 

 of posterior end, with a short spine, is found in P. stevensoni (Metcalf, 

 1923a, fig. 26), and this appears also, by the way, in Cepedea spinifera 

 (Metcalf, 1923a, fig. 104). 



Cepedea has the bent spindle form of its immediate ancestor 

 Protoopalina, but we do not find the markedly divergent types of 

 posterior end. We do, however, find some species with two types of 

 body, one rather slender, the other stocky. Whenever a species of 

 Cepedea or of Protoopalina shows these two types, intermediate forms 

 in the same species, and usually in the same infections, will be found, 

 the same individual host containing all three — slender, broad, and 

 intermediate. More or less faint indications of a posterior hook or 

 spine appear in some Cepedeas, for example, in C. dimidiata forma 

 zelleri (see Metcalf, 1923a, fig. 105, middle, left-hand drawing). 

 In some Opalinas a posterior spine or faintly indicated hook may 

 appear, and in some Zelleriellas the asymmetrical, posterior hook 

 may be greatly developed, for example, in Z. antunesi (fig. 42); and 

 in the degree of development of any of these features individual 

 opalinids as well as species difl'er. 



This all makes taxonomic conclusions uncertain until after study of 

 much material from many hosts. Without such intensive study, 

 specific diagnosis must be tentative. 



The two flattened genera, Zelleriella and Opalina, each show in 

 many individuals of many species mdication of a bend in the anterior 



