Body Symmetry: over the course of evolution radial and bilateral symmetry is developed.

Cephalization: is the tendency to concentrate the nerves and organs towards the front of the body

Coelom: the development of a jelly like layer between the ectoderm and endoderm tissues

The first multicellular animals, which are discovered and examined using Ediacaran fossils, exhibit a flat or plate-shaped surface made of soft tissues. They have segmented bodies and display a bilateral symmetry. It turned out that these animals live on sea floor of shallow oceans.

On the other hand, Burgess Shale fossils show some differences when compared to the fossils of the first multicellular animals like the Ediacaran fossils. They have gone through evolution when the Cambrian period started. These changes were discovered when scientists examined the shells, skeletons, and other body parts of the Burgess Shale fossils. From the soft-bodied tissues of the Ediacaran fauna, the Burgess Shale animals exhibit hard exoskeleton and forelimbs that are used to catch their prey.

Burgess Shale fossils show some differences when compared to the fossils of the first multicellular animals like the Ediacaran fossils. They have gone through evolution when the Cambrian period started. These changes were discovered when scientists examined the shells, skeletons, and other body parts of the Burgess Shale fossils. From the soft-bodied tissues of the Ediacaran fauna, the Burgess Shale animals exhibit hard exoskeleton and forelimbs that are used to catch their prey.

Unlike flatworms, modern invertebrates have a more complex organization beginning with specialized cells that form tissues, organs, and organ systems. Because of the evolution, modern invertebrates are now using different organ systems to perform more complex functions. They are now able to respond better to the environment due to the development of some organs such as the brain and the coelom.

Here is the comparison between the body structures and characteristics of cnidarians and mollusks:

Cnidarians:
a. exhibits radial symmetry
b. absence of cephalization
c. absence of coelom (acoelomates)
d. body cavity has one opening that acts as mouth and anus
e. two germ layers are present
f. appears in two body forms: polyp and medusa
g. presence of cnidocytes (for capturing prey)

Mollusks
a. exhibits bilateral symmetry
b. presence of cephalization
c. exhibits true coelom
d. protostomes
e. three germ layers are present
f. body has a mantle, muscular foot or tentacles, visceral mass, shells

The fossil of Marella splendens exhibits a large spinous exoskeleton, two pairs of antennae, feathery gills, and a segmented body with a pair of identical appendages. This fossil could be closely related to crustaceans and chelicerates since they have similarities in the anatomical structure.