ONE-CELLED ANIMALS OR PROTOZOA 45 



When an animal has no free larva, but quits 

 the egg in a form practically identical with that 

 of the adult, the development is said to be 

 "direct." But changes equally startling with 

 those displayed when a larva develops into the 

 adult form, may take place while the young ani- 

 mal is enclosed within the egg itself. To these 

 also zoologists apply the axiom referred to above, 

 that the history of the individual summarises the 

 history of the race. Thus, for example, the Am- 

 phibian larva, e.g. the tadpole of a frog (p. 153) 

 has gills, which disappear in the adult form : the 

 young reptile, bird, or mammal, which has no 

 larval stage, has gills during a comparatively 

 early stage ; and loses them at a later period of 

 its development. In each case zoologists con- 

 clude that the animal is descended from a fish- 

 like ancestor, which possessed gills all its life, 

 and that the more immediate ancestors in the 

 family tree, have lost their gills by degrees. 



The study of the progressive changes of 

 young forms, whether larval, or enclosed within 

 the egg, is called Embryology, and constitutes, in 

 these days, the major branch of zoological sci- 

 ence. That it is of paramount importance to the 

 student of classification, engaged upon the sort- 

 ing of the animal kingdom, will be apparent from 

 what has been stated above. 



CHAPTER IV 



THE ONE-CELLED ANIMALS OR PROTOZOA 



SOME idea of the general characteristics of the 

 Protozoa has already been given by the descrip- 

 tion of Amoeba. We may now say something 



