1. The ancestral multicellular organisms diverge; some develop true tissues (Eumetazoa) while others do not (Parazoa).
   
2. Radial symmetry means that the animal has a top and a bottom; these animals have a mouth and an aboral side but no head, rear, left or right.  Radials are usually attached (sessile) or drifting.  You have bilateral symmetry: a top and bottom (dorsal and ventral), a head and tail (anterior and posterior), and a left and right.  Bilaterals have an evolutionary trend of cephalization, or increasing sensory organs on the anterior (head) side; perhaps due to its being the first to encounter stimuli.
   
   Embryos have three germination layers in bilaterals and two in radials.  In bilaterial animals an ectoderm, endoderm, and mesoderm exist; that is why they are called "triploblastic":  bilaterial blastula have three types of tissue.  The mesoderm forms the muscles and most of the other tissues.  In radial animals an ectoderm and endoderm exist.  The endoderm lines the rudimentary gut (archenteron) of the animal and the ectoderm is the outer covering.  Therefore radials are diploblastic.
   

3. Acoelomates are triploblastics with solid bodies. From inside, out the order is digestive cavity, digestrive tract, tissue, covering - there are no cavities.
   
   All other triploblastics:
   
   
   The pseudocoelom is the body cavity not completely lined by mesoderm-derived tissue.  The coelom is the fluid-filled cavity completely lined by tissue originating in the mesoderm.  The mesentary is the mesoderm connection dorsally and ventrally that suspends the organs in the body cavity known as the coelom.  Body cavity functions include cushioning, organ growth and movement; and in soft bodies, it functions as a hydrostatic skeleton for muscles.  In other words, it acts as a rigid bone substitute in non-bony organisms.
   

4. The difference between Protostomes and Deuterostomes is three-fold:  cleavage, blastopore fate and coelom formation.  Protostomes undergo spiral cleavage diagonal to embryo's vertical axis.  The cleavage is also determinate, meaning the fate (i.e., muscle, nerve, etc.) of the cells occurred very early in the embryonic condition.  Deuterostomes on the other hand have radial indeterminate cleavage parallel or perpendicular to the vertical axis; any embryonic cell has the potential to produce its own embryo if separated from the other embryonic cells, making identical twins possible.
   
   Blastopore fate.  Protos and Deuteros differ in from which opening the mouth forms.  The blastopore is the first opening; the second opening forms on the other end to form a digestive tube with mouth and anus.  Protostome mouths originate at the blastopore while deuterostome mouths originate at the second opening.
   
   
   Coelom formation.  In protostomes as the archenteron is forming the solid mass of mesoderm is split by the invagination.  This is called schizocoelous development.  In deuterostomes the mesoderm actually buds from the archenteron wall and hollows to form the coelom cavities through the process of enterocoelus development.
   

Next:  "Sponges."