Every animal which is made up of many cells, has to exhibit a body plan which solves certain problems of existence. Each organism must be able to capture food and excrete the waste (digestive system). Each must have some sort of communication between cells or body parts (nervous system). Each must access oxygen and get rid of carbon dioxide (gas exchange). Each must have some sort of protection against the environment and predators. Each must sense the environment to the extent that it can capture food and avoid predators. And each must be able to produce offspring. When one considers all the invertebrates (animals without backbones) with their wildly different solutions to these problems, as well as the vertebrates (animals with backbones) which are more alike, it is evident that there are some wildly different body plans.
Each distinct animal group exhibits a unique body plan. There are starfish, for example, which have their bodies divided into five equal parts (or multiples of five). This group includes sea urchins, ‘sand dollars’ and crinoids or ‘sea lilies’ which have the body proper positioned on top of a flexible stalk (so that the whole thing looks like a tulip or lily, hence the name, but they are animals none-the-less). Then there are snails with twisted bodies under a shell positioned over a strong muscular foot and slugs with straight bodies but no shell. These form a distinguishable group called gastropods or ‘mouth-foot’. Then there are clams, oysters and scallops with 2 shells connected by a hinge. They are called bivalves and they are a distinguishable group. The body plan of the arthropods (animals with a jointed exterior skeleton made of chitin) are particularly remarkable. In this group we find insects, crustaceans, arachnids like spiders, and extinct creatures like trilobites.
You get the idea. Animal body plans follow set patterns related to geometry of development or symmetry. They may show no pattern at all, such as sponges, be radial (like starfish) or develop from 2 mirror halves such as lobsters or animals with backbones. Animals thus can be assigned to one or other of some obviously unique body plans. It seems like a good question to ask where these animals with distinct body plans came from.
Two answers come to mind. Either God created them in separate groups, or all of them developed gradually from much smaller, much simpler organisms that over time became larger and more complicated and more different from each other. The coordination of vastly different body parts into an integrated and successful whole, is a clear indication of design. As Stephen Meyer remarked in Darwin’s Doubt “To build a new animal and establish its body plan, proteins need to be organized into higher-level structures. In other words, once new proteins arise, something must arrange them to play their parts in distinctive cell types. These distinctive cell types must, in turn, be organized to form distinctive tissues, organs, and body plans. This process of organization occurs during embryological development. Thus, to explain how animals are actually built from smaller protein components, scientists must understand the process of embryological development.” [p. 257]
The bottom line is that animal body plans are exquisitely organized in a most amazingly diverse collection of forms and lifestyles. Since Cambrian explosion studies have revealed no ancestral connections between these various body plans, it is evident that evolutionary explanations are useless to explain what we see in the animal world. The sudden appearance in the fossil record of wonderful and functional designs really does display the work of God.