Animals in which a true body cavity or coelom is completely absent are known by which of the following biological terms?

Introduction / Context:
In zoology, the type of body cavity present in an animal is an important feature used for classification. Textbooks describe three main categories based on the presence or absence of a coelom, which is a fluid filled body cavity lined by mesoderm. Many entrance and competitive exams ask candidates to distinguish between coelomates, pseudocoelomates, and acoelomates. This question focuses on the group of animals that completely lack a body cavity.

Given Data / Assumptions:

• The question asks about animals in which a body cavity is absent.
• The term body cavity refers to the coelom.
• The options list different zoological terms, including coelomates, pseudocoelomates, and acoelomates.
• We assume standard definitions used in introductory zoology.

Concept / Approach:
Animals can be grouped based on the presence of a coelom. Coelomates have a true coelom completely lined by mesoderm. Pseudocoelomates have a cavity not fully lined by mesoderm, which is called a pseudocoel. Acoelomates have no body cavity between the gut and the body wall; their body is solid except for the digestive cavity. Classic examples of acoelomates are flatworms such as planaria. Therefore, the correct approach is to identify which option names animals lacking a body cavity as acoelomates.

Step-by-Step Solution:
Step 1: Recall the three main terms: coelomates, pseudocoelomates, and acoelomates. Step 2: Understand that coelomates possess a true coelom, pseudocoelomates possess a false coelom, and acoelomates lack a coelom. Step 3: Match the definition in the question, which clearly states that the body cavity is absent. Step 4: Recognize that acoelomates literally means animals without a coelom. Step 5: Select acoelomates from the options as the correct term.

Verification / Alternative check:
To verify, consider examples. Flatworms, which belong to phylum Platyhelminthes, are described in textbooks as acoelomates because they have no cavity between the digestive tract and the body wall. In contrast, earthworms and humans are coelomates, while roundworms such as Ascaris are pseudocoelomates. These examples confirm that the correct term for animals lacking a body cavity is acoelomates and not any of the other options given.

Why Other Options Are Wrong:
Option A: Coelomates have a true coelom and therefore do not match the definition of animals with no body cavity. Option B: Wild is a nontechnical term describing animals living in nature and has nothing to do with body cavity classification. Option C: Pseudocoelomates have a false or partially lined body cavity, not an absent one, so this does not answer the question. Option E: Vertebrates is a very broad group which usually includes coelomate animals with a well developed body cavity, so it is not correct here.

Common Pitfalls:
Students sometimes mix up the spelling and meaning of coelomate related terms because they look similar. Another common error is to assume that pseudocoelomates are the same as acoelomates. The key is to remember the prefixes: a indicates absence, pseudo indicates false, and the base word coelom refers to a cavity. Learning simple examples, such as flatworms as acoelomates and roundworms as pseudocoelomates, can greatly reduce confusion during exams.

Final Answer:
Animals in which the body cavity or coelom is completely absent are known as acoelomates.