Classification can be done through the development of the germ layers of the embryo.
Germ Layer Theory. See image here
The 3 germ layers consist of
1. Outermost layer of early embryo - ectoderm
2. Middle layer of early embryo - mesoderm
3. Innermost layer of early embryo - endoderm
Ectoderm of early embryo develops to form the skin epidermis and tooth enamel. It also differentiates to form the nervous system - consisting of nerve cells known as neurones that make up the spinal cord and the brain.
Mesoderm which is the middle layer of the early embryo, has a great capacity to differentiate into a number of tissues and structures including bone, cartilage, muscle, connective tissue, reproductive, excretory and secretory systems. The mesoderm in large and complex organisms gives rise to a coelom, ie body cavity that contains the body organs.
The innermost layer which is known as the endoderm which is first columnar, differentiates to form epithelial cells of the various organ systems, eg gastrointestinal system, respiratory tract, endocrine system, auditory system and urinogenital system.
One exception which does not have three layers of body tissue would be the unicellular protozoa. Other radially symmetrical organisms such as the Phylum Cnidaria and Phylum Porifera have only 2 layers consisting of the ectoderm and endoderm.
Other phyla which have the three germ layers have developed special features. Eg. Phylum Chordata has developed a spinal cord, phylum arthropods developed a tough, chitinous exoskeleton
Activity Four
Google and write down examples of three species from two phyla of the Kingdom Animalia that interest you.