Characteristics of the Protozoa. clxi 



as is the case with the parasitic Gregarinae, and absorb soluble 

 pabulum at all points of its exterior. In other cases the pseudo- 

 podia, in which a constant cyclosis of the granular cytoplasma may 

 be observed to be carried on between the central part of the animaFs 

 body and its radial processes^ act as suctorial and absorbing organs. 

 In one class only, the Infusoria, do we find both mouth and anus ; 

 but these two orifices are not connected with each other by any conti- 

 nuous tubular canal, the ingested aliment passing from the mouth and 

 oeso^^hagus into the general parenchyma of the body, and the refuse 

 matter finding its way into the neighbourhood of the anus, whence 

 it is extruded by the contractile sarcodic substance surrounding it. 

 In many Protozoa the vacuolation of the contractile protoplasm 

 produces the structures known as ' contractile ' or ' pulsatile vesi- 

 cles,' which have sometimes been regarded as a rudimentary circula- 

 tory apparatus, but which, as they have sometimes been observed 

 to open externally, may with more probability be considered to 

 be depuratory in function, and to correspond with the water-vas- 

 cular system of higher animals. No specialized respiratory nor 

 nervous system exists in this sub-kingdom, unless the red pigment- 

 sj)ecks of certain Infusoria may be considered to correspond to 

 eyes. 



Reproduction is ordinarily asexual, taking place in the ways of 

 fission and gemmation, but in the Spongiadae and Infusoria we find 

 true sexual reproduction by means of ova and spermatozoa. Encys- 

 tation very frequently accompanies the agamogenetic, and conju- 

 gation the sexual process. 



The Protozoa may be either solitaiy or social, and are found 

 either in sea or fresh water, or as Entozoa, but their respiration is 

 never aerial. 



The Gregarinae would by most writers be considered, as they are here, 

 to be the lowest of the Protozoa. Their ento-parasitic habits, however, 

 which will account for nmch of the simplicity or degradation of tlieir 

 organism, must not cause us to overlook their close affinity to certain 

 forms of Rliizopoda, especially the Amoehina; and it has been rather 

 from considerations of convenience, which, in the absence of any actual 

 demonstration of genetic affinity, have weight in classification, that they 

 have been here separated from that class. The Rliizopoda are by some 

 writers placed higher, by others lower, in the scale of life than the Infu- 



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