The ancestral multicellular organisms diverge; some develop true tissues
(Eumetazoa) while others do not (Parazoa).
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.
Acoelomates are triploblastics with solid bodies. From inside, out the order
is digestive cavity, digestrive tract, tissue, covering - there are no
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.
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
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
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