ORDER CILIO-FLAGELLATA. 441 



boundaries for the admission of diverse other types which maintain under variously 

 modified conditions the characteristics of the two larger sections of the Flagellata- 

 Eustomata and ordinary Ciliata, and thus intimately connect together these, at first 

 sight, seemingly altogether independent groups. In illustration of a few of the more 

 prominent relationships that are established between the two series of the typical 

 Flagellata and Ciliata through the intermedium of the present order, it may be sub- 

 mitted that the retraction of the single flagellum is alone needed, in the genera 

 Stephanomonas and Asthmatos, to convert these types into simple Peritricha resembling 

 Strombidium, though in the case of Asthmatos it is worthy of remark that the con- 

 stituent elements of the terminal tuft of cilia are so long as to partake almost of the 

 nature of flagella, thus conveying to the animalcules an aspect closely corresponding 

 with that of the multiflagellate type Lophomonas. In Trichonema and Mitophora a 

 similar suppression of the single flagellate appendage would transform these animal- 

 cules into simple Holotricha ; while in the case of Heteromastix, where the flagella 

 constitute the most important organs of locomotion, it requires only the obliteration 

 of the adoral ciliary fringe to produce a typical representative of the simply flagellate 

 genus Heteronema. The affinities of the important family group of the Peridiniidae 

 are discussed at length in connection with the description given of that earliest 

 recognized and typical section of the Cilio-Flagellata. 



K. M. Diesing, in his ' Revision der Prothelminthen,' * distinguishes this Cilio- 

 Flagellate order of the Infusoria by the title of the Mastigophora Trichosomata. 



An analytical key to the various families and genera of the Cilio-Flagellata, as 

 here delimited, is given on the opposite page. 



Fam. I. PERIDINTIDJE, Ehr. 



Animalcules free-swimming, persistent in form, sometimes naked, but 

 mostly invested by an indurated carapace or cuirass ; flagellum usually 

 single, ciliary system constituting a more or less perfectly developed zone- 

 like girdle ; oral aperture distinct ; an eye-like pigment-spot frequently 

 developed. Inhabiting salt and fresh water, often highly phosphorescent. 

 Increasing by fission and by subdivision into sporular elements. 



The representatives of this highly characteristic family group are readily distin- 

 guished by the girdle-like disposition of the cilia which supplement the customarily 

 single, but in one instance double flagellate appendage. It is further found that 

 the body is with but few exceptions enclosed within a densely indurated carapace 

 or cuirass, which is itself frequently composed of a greater or less number of poly- 

 gonal and often elegantly sculptured facets. By far the larger number of species 

 are inhabitants of salt water, agreeing in this respect with the pelagic Noctilucce^ 

 with which they further correspond, in many instances, by the exhibition of brilliant 

 phosphoric properties. Excepting for the presence of the equatorial ciliary girdle it 

 may be further noted that in such a non-encuirassed form as Gymnodinium the 

 resemblance of external contour to that of Noctiluca is so conspicuous including 

 the possession in each case of a corresponding and apparently homologously 

 developed non-ciliated vertical or ventral groove that it would not seem incon- 

 sistent to anticipate that forms will yet be met with that still more closely connect 

 these two types with one another. In no representative of the Peridiniidae, 

 however, as yet examined, has the internal body-substance or endoplasm been 

 observed to exhibit that peculiar vacuolar or reticulate character so eminently dis- 

 tinctive of the Noctilucidse. In the possession, more especially by the fresh-water 

 members of this group, of a frequently brilliant scarlet eye-like pigment-spot, 

 combined with the pervading green or other coloured hue of the general body-sub- 

 stance, some distant affinity with the family of the Euglenidae would seem to be 



* ' Sitz. der Kaiser. Akad. der Wiss. Wien,' 1865. 



