Question

Which of the following does not occur? a. radially symmetrical diploblast b. diploblastic eucoelomate c. protostomic coelomate d. bilaterally symmetrical deuterostome

Short answer

The combination that does not occur is a diploblastic eucoelomate.

Step-by-step solution

Eliminate the correct combinations

Some of these combinations of characteristics are recognized in specific groups of organisms. For example, a 'radially symmetrical diploblast' correctly describes cnidarians, like jellyfish. Also, a 'protostomic coelomate' correctly describes many types of invertebrates, such as mollusks and annelid worms. Finally, a 'bilaterally symmetrical deuterostome' correctly describes many types of chordates, which includes all vertebrates.

Identify the incorrect combination

Diploblastic organisms only have two germ layers, the ectoderm and endoderm, which is only enough to give rise to tissues like the outer coverings and the digestive tract. A true coelom, or eucoelom, is a body cavity that is completely lined with mesoderm tissue. Diploblasts lack a mesoderm, and so they cannot form a true coelom.

Key concepts

Radial Symmetry

Radial symmetry is a fascinating and distinct form of symmetry found in nature. It means that the body of an organism is arranged around a central axis.
Imagine a circle. You can slice through the center at any angle and both halves would look the same. That's very similar to radial symmetry.

This type of symmetry is common in organisms like jellyfish, sea anemones, and starfish at certain life stages. These creatures are often termed diploblastic because they are made up of two primary germ layers: the ectoderm and the endoderm.
While radial symmetry provides simplicity, it also offers some advantages. For instance, organisms can interact with their environment from all sides equally. Cnidarians, such as jellyfish, are prime examples of this structure. They are well adapted to a drifting, aquatic lifestyle thanks to their symmetrical design.

Bilaterally Symmetrical Deuterostomes

Bilateral symmetry is another form of symmetry in the animal kingdom. Here, the body is divided into two mirror-image halves along a single plane, typically running from head to tail.
This form of symmetry is associated with the evolution of a head, contributing to the complexity and functionality of organisms. Organisms with bilateral symmetry usually exhibit a phenomenon called cephalization. This is the formation of a head where sensory organs are concentrated, allowing for more directed and efficient movement.

Deuterostomes are a group of animals first identified during their embryonic development. The word comes from Greek, where "deutero" means second and "stome" means mouth. This naming reflects their unique developmental process, where the initial opening in the developing embryo becomes the anus, and the mouth forms second.
Bilaterally symmetrical deuterostomes include many familiar animals, offering great diversity. Chordates, which encompass all vertebrates like mammals, birds, and fish, are notable examples.

Diploblasts and Triploblasts

In animal development, germ layers play a crucial role. These layers form during the embryonic phase and differentiate into various tissues and organs.

Diploblasts are organisms that have two germ layers: the ectoderm and endoderm. They do not possess a middle layer, known as the mesoderm. Their simplicity is exemplified by cnidarians, which don't have complex organs. Their structure supports basic functions and direct interactions with their environment.

• Ectoderm: Forms the outer layer and becomes skin, nerves, and forms the nervous system.
• Endoderm: Forms the inner layer, developing into digestive tissues and structures.

Triploblasts, on the other hand, boast three germ layers. These layers are the ectoderm, mesoderm, and endoderm.
The presence of the mesoderm in triploblasts allows for more complex structures, like muscles and a true coelom. The triploblast arrangement gave rise to more diverse body plans and complex organisms, enhancing their adaptability. It's a significant evolutionary step that enabled the development of organs and systems, such as those found in humans.