THE OPALINID GILIATE IXFUSORIAXS. 263 



in form, the combination of form characters in the several infections 

 compared is very helpful in indicating specific relations. 



Size of hody must be used with caution. Here again we must 

 compare infections as a whole, for the daughter cells are smaller 

 than full-grown forms. We must be careful, also, to compare truly 

 comparable infections, from corresponding phases of the life cycle. 



CUiation. — Length of cilia in comparable individuals seems a 

 usable character. We may say the same, in general, of density 

 of ciliation. Zelleriella hirsuta, for example, is far more densely- 

 ciliated than other species. But one character which we have 

 measured and stated for almost all the species described must be 

 warily used. This is the width of the interval between the lines of 

 cilia at the anterior end of the body and at its posterior end. Prob- 

 ably the interval between the lines of cilia varies somewhat with the 

 nutrition of the individual, well-nourished, fat individuals perhaps 

 having the lines of cilia further apart than they would be in the 

 same individuals if starved and shrunken. Nearly all species of 

 Opalinidae have main lines of cilia running the whole length of the 

 bod}', with other, shorter lines interpolated between them in the an- 

 terior portion of the body. In general, the lines of cilia are about 

 twice as numerous at the anterior end as at the posterior end. In 

 some species, on the other hand, they are about three times as numer- 

 ous. In a few species they are about four times as numerous in front. 

 But here again caution must be exercised. After transverse fission 

 a posterior daughter may have, for a time, fewer anterior lines of 

 cilia than would a full-grown form. The number of cilia to the 

 micromillimeter in a line of cilia is probably a fairly reliable char- 

 acter for specific distinction. The basal granules stain well with the 

 hematoxylin strains and can often be counted under high magnifica- 

 tion without unreasonable difficulty. The basal granules in any line 

 of cilia are, in general, somewhat closer together in the anterior part 

 of the body than they are posteriorly, so corresponding portions of 

 the body must be compared, and one must be careful to compare full- 

 grown individuals and not daughter cells. Unfortunately an acci- 

 dentally contaminated mounting medium caused most of my slides 

 to fade before I got to the study of this point, so I have not used 

 these probably useful measurements. 



Relative proportions of ectosarc and endosarc. In comparing cer- 

 tain species this character is useful, but we must remember that in 

 some species, such as Protoopalina saturncdis (fig. 37, p. G4), which 

 have individuals of two forms, one slender, the other swollen, the 

 relatiA^e proportions of the ectosarc and endosarc differ in the two 

 sorts of individuals. In making comparisons we must be careful to 

 choose corresponding individuals. The thickness of the ectosarc is 

 much less at the front end of the body in some species, while in others 



