THE CLASSIFICATION OF ANIMALS. 129 



sesses, on the contrary, a fixed outline, a definite channel for 

 the entrance of food — the " gullet," and a fringe of cilia. The 

 latter are small — equal-sized permanent processes of proto- 

 plasm, which by their vibration create a current in the water, 

 and drive food into the " gullet." Vacuoles of ingestion and 

 of excretion are seen as in Amaiba. Reproduction may take 

 place by fission into two equal halves, of which, one remains 

 fixed, while tbe other is a free-swimming form, provided at 

 the base with an extra ring of cilia instead of a stalk. It 

 may also take place by what is spoken of as " spore forma- 

 tion " or as " gemmation (i.e. budding) from the endoplast," 

 the nucleus breaking up into a number of parts, from. wMch 

 originate small free-swimming forms. A small form is also 

 sometimes produced by unequal or by repeated fission of one 

 of the ordinary stalked forms. Conjugation is observed to 

 take place between one of the small forms and an indivi- 

 dual of the ordinary stalked form, the former being absorbed 

 into the latter. 



Position ats a type of its sub-hingdom. Vorticella, as has been 

 already stated, afEords an instance of the highest degree of 

 differentiation attained by single cells. It is accordingly one 

 of the higher types of its sub-kingdom. 



Habits. Vorticella is a minute animal just distinguishable 

 by the naked eye. It is a fresh-water form, found in ponds, 

 fixed on weeds, sticks, and animal organisms. 



Development. The small free forms produced from "spores'" 

 become fixed in time, losing the lower ring of cilia by means 

 of which they swim, and developing a stalk instead; they 

 then, grow till they attain the usual size. 



Place in classification. Sub-kingdom Protozoa ; class 

 iNrusORlA ; order Giliata ; sub-order Peritricha ; family Vnrti- 

 cellidcB ; genus Vorticella (see p. 302). 



Detailed description. See Huxley and Martin's " Practical 

 Biology"; Parker's "Elementary Biology"; and Howes' 

 " Atlas of Practical Elementary Biology." 



