3 88 



ELEMENTARY ZOOLOGY. 



Gyloniai ^Protozoa, 



oa 



FIG. 1 60. A diagram to suggest the way in which individuals of the different 

 phyla have parallel development for a while, and later diverge in their own particular 

 way. For example, an Amphibian begins life as a single cell, similar to the simple 

 protozoa (i) ; later it forms a mass of cells similar to some of the colonial protozoa (2) . 

 in forming a gastrula (3) it is going through a stage similar to the Hydra, which is a 

 kind of permanent gastrula. Amphibians run parallel to the fishes in development, in 

 the formation of a body cavity (4) , in forming segments (5) , in development of notochord 

 (6), in the presence of gills in early life (7); but part company with the fishes in the 

 fact that they lose the gills and develop lungs (8). 



Questions on the Figure. Trace out other instances of parallelism 

 until the purpose and meaning of the diagram become clear. Then 

 find other facts with respect to development of animals and show 

 how the diagram cannot do justice to the whole truth. Compare 

 diagram with text, sections 385 to 387. 



