40 ZOOLOGY. 
Crass IV.—INFUSORIA. 
Flagellate or ciliate (sometimes only ciliate in the early stages) Protozoa, 
the body not changing in form, having a definite skin, and often wholly or 
partly provided with cilia; usually free, sometimes stalked or attached; with 
a@ mouth-opening and esophagus, and rudiments of digestive, circula- 
tory (two or more contractile vesicles), and reproductive organs (nucleus and 
nucleolus), but with no distinctively sexual organs. 
Order 1, Flagellata.—Rounded, oval, or pear-shaped organisms, usually 
exceedingly minute, provided with one or two flagella, with 
an oral region, into which particles of food are thrown by 
the flagellum ; with a nucleus and contractile vesicles, rarely 
stalked, and with a calyx; sometimes aggregated; witha row 
of cilia in the highest forms serving as a locomotive appara- 
tus ; reproducing by self-division or by segmentation of the 
protoplasmic contents of the body, the young being minute 
oval bodies, provided with a flagellum (Monas, Heteromita, 
Noctiluca, Peridinium). 
Order 2. Tentaculifera (Suctoria).—Naked, not ciliated, protozoans, 
with long, stiff, retractile arms or tentacles, provided with 
a sucker at the end, the arms hollow, conveying the food 
to the digestive cavity ; originating from ciliated young ; 
also by self-division throwing off ciliated forms, and under- 
going conjugation (Acineta). 
Order 8. Oiliata (True Infusoria).—Body free and covered with cilia 
(Paramecium, Stentor, etc.), or stalked, with the cilia con- 
fined to the head end (Vaginicola and Vorticella, etc.); a 
well-defined mouth and cesophagus ; a digestive cavity and 
vent ; a large nucleus, and two or more contractile vesicles. 
Reproducing by self-division, budding or conjugating, and 
producing monad-like young by self-division of the nu- 
cleus ; sexuality doubtfully indicated. 
The following diagram represents the relative position 
of the orders and classes of Protozoa, and in arude way 
their possible genetic relations : 
vertebrated Animals, p. 662), “‘ it is still possible that the conjugation 
of the Infusoria may bea true sexual process, and that a portion of the 
divided endoplastules [striated nucleoli] of each may play the part of 
the spermatic corpuscle, the conjugation of which with the nucleus of 
the ovum appears, from recent researches, to constitute the essence 
of the act of impregnation.” See also Hartog in Cont, Review, 1892, 
