Explain by what principle the sciences studying zoology are subdivided?

Zoology is a science that studies the external structure, internal structure, behavior, ecological characteristics, distribution of animals. In turn, zoology is divided into several sciences: animal morphology, animal anatomy, animal ecology, and others. According to the objects of study, zoology is divided into many specialized sciences: ornithology (the science that studies birds), herpeology (the science that studies reptiles and amphibians), ichthyology studies fish, theriology studies mammals, entomology studies insects, helminthology studies parasitic worms.