18 



LECTURE IT. 



malcules ; they have been termed from this easy and common mode 

 of procuring them, the animals of infusion, or Infusoria. 



The earlier microscopi- 

 5 cal observers confounded 



all the minute living ob- 

 jects which they thus met 

 with under that term ; 

 but the progressively in- 

 creasing powers of defini- 

 tion at the command of 

 later observers have led to 

 the removal of the single- 

 celled locomotive plants 

 {Jig. 7.), together with 

 the embryos of polypes, 

 worms, and insects, from 

 this motley and heteroge- 

 neous group, and have re- 

 stricted it to those ani- 

 mals which, in their fully 

 developed state, mani- 

 fest a form of body more 

 or less amorphous, devoid 

 of radiated prehensile arms, without definite locomotive members, 



commonly moving by means 

 of minute superficial vibra- 

 tile cilia more or less dif- 

 fused over the surface of 

 the body, or aggregated in 

 circular groups near the 

 head, where they produce 

 by their successive oblique 

 action the appearance of 

 rapidly rotating wheels. 

 Only the largest species of 

 Infusoria are provided with 

 the last specified arrange- 

 ment of vibratile cilia, and 

 these " wheel-animacules," 

 as they are termed, being 

 endowed with a higher 

 type of organisation, more 

 especially of the digestive system, constitute a distinct class of Infu- 



Vorticella nebulifera. 



I, 2. Spirostomum vircns ; 3—8. Glaucoma scintillans 



