40 ZOOLOGY. 



Class IV. — In-pusoria. 



Flagellate or ciliate (sometimes only ciliate in tlte eaHy stages) Protozoa, 

 the hody not cha/nging inform, having a definite skin, and often wholly or 

 farUy provided with cilia; usvMly free, sometimes stalked or attached; witli 

 a mouth-opening and (esophagus, and rudiments of digestive, circular- 

 iory (two or more contractile vesicles), and reproductive organs (nucleus and 

 wucleolus), but with no distinctively sexual organs. 



Order 1. MageUata. — 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; with a 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, 

 Koctiluca, Peridinium). 



&rder 2. Tenta^ulifera (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 ofE ciliated forms, and undergoing 

 conjugation (Acineta). 



Order Z. OHiata (True Infusoria). — Body free and covered with cilia 

 (Paramecium, Stentor, etc.), or stalked, with the cilia con- 

 fined to the head end (Vaginicola and Voi-ticella, etc.) ; a 

 well-defined mouth aod 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 a rude way 

 their possible genetic relations : 



vertebrated Animals, p. 663), " it is still possible that the conjugation 

 of the Infusoria may be a 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." 



