Question

Which is the correct hierarchy of divisions in the animal kingdom, from most inclusive to least inclusive? a. Protostomia, Ecdysozoa, Bilateria b. Ecdysozoa, Protostomia, Bilateria c. Bilateria, Protostomia, Ecdysozoa d. Protostomia, Bilateria, Ecdysozoa e. none of the above

Short answer

Correct answer: c. Bilateria, Protostomia, Ecdysozoa. Bilateria is the most inclusive group, followed by Protostomia, and then Ecdysozoa is the least inclusive.

Step-by-step solution

Understand the biological terms

The three biological terms to understand are:\- Protostomia: This is a clade of animals. It includes creatures that undergo a specific development from the blastula stage, which leads to the mouth before the anus during their lifecycle.\- Ecdysozoa: This is a group of protostomes, which are characterized by their ability to molt their exoskeleton.\- Bilateria: This is a major group of animals. They have bilateral symmetry, i.e., they have a front and a back end, as well as an upside and downside.

Determine the order of inclusivity

Based on the definitions and understanding of the biological concepts:\- 'Bilateria' is the most inclusive group, as it includes most animals.\- 'Protostomia' is less inclusive than 'Bilateria', as it is a subset based on a specific kind of development.\- 'Ecdysozoa' is the least inclusive, as it consists only of those protostomes, which molt their exoskeleton.

Select the correct option

Evaluate each option according to the order of most inclusive to least inclusive determined in Step 2. The option that follows the correct order, Bilateria > Protostomia > Ecdysozoa, is the correct answer.

Key concepts

Bilateria

The Bilateria are a large and complex grouping within the animal kingdom that demonstrates bilateral symmetry. This means that if you draw a line down the middle of the organism, from head to tail, the two halves would mirror each other. Most animals, including humans, are part of this group. Bilaterians are more developed compared to their counterparts like sponges and jellyfish that belong to the Radialia, exhibiting radial symmetry.

Bilaterians also have three distinct germ layers - the ectoderm, mesoderm, and endoderm - leading to the development of more complex structures such as a digestive system with a true stomach and intestines. This increased organization reflects the complexity of the organisms within the Bilateria group and explains why they are the most inclusive category in this hierarchical context. They are fundamentally divided into two groups: Protostomia and Deuterostomia, based on their embryonic development.

Protostomia

Protostomia, a subset of Bilateria, are characterized predominantly by their embryonic development. The name itself translates to 'mouth first,' referring to the development of the mouth before the anus during the animal's early growth stages. This feature is key in distinguishing Protostomia from Deuterostomia, where the opposite sequence occurs during development.

Protostomes are further categorized based on their body cavities into groups such as the lophotrochozoans, which include mollusks and annelids, and the ecdysozoans, which we will cover next. Protostomes are less inclusive than Bilateria because they make up one of the major developmental pathways that bilaterian animals can take. Understanding the embryological development of organisms within this clade can provide insights into their evolutionary relationships.

Ecdysozoa

Ecdysozoa represents a categorically distinct group within the Protostomes, recognized primarily for one common trait: ecdysis, or molting. Ecdysozoans shed their external layer as they grow, a phenomenon observed in arthropods like spiders and insects, as well as in nematodes and other related groups.

Ecdysis is essential for these organisms because their rigid exoskeletons do not grow with their bodies. To accommodate growth, they must periodically molt their outer layer to increase in size. Ecdysozoans account for a vast number of species, however, they are the least inclusive of the three groups we are considering because their defining characteristic is more specialized, applying to a narrower range of organisms within the Protostomia.