In animal body organisation, the internal body cavity that is completely lined on all sides by mesodermal tissue is called __________.

Introduction / Context:
This biology question tests your understanding of basic animal body organisation, specifically the concept of a body cavity and how it is lined. In zoology, animals are often classified as acoelomate, pseudocoelomate, or coelomate based on the presence and nature of a body cavity. The term for a body cavity that is completely lined by mesoderm is an important definition used throughout comparative anatomy and classification of animals.

Given Data / Assumptions:
- The question refers to an internal body cavity. - It clearly mentions that this cavity is lined by mesoderm. - The options include a structural term and names of animal phyla. - Standard zoology definitions for body cavities are assumed.

Concept / Approach:
In triploblastic animals, three germ layers form during embryonic development: ectoderm, mesoderm, and endoderm. The type of body cavity between the body wall and the gut is a key feature. When there is no body cavity, the animals are called acoelomates. When the cavity is not completely lined by mesoderm, it is a pseudocoel, seen in pseudocoelomates. When a true cavity is present and it is completely lined by mesodermal epithelium, it is called a coelom, and such animals are called coelomates. The term coelom refers specifically to this true body cavity and is used across many phyla such as Annelida, Arthropoda, Mollusca, and Chordata.

Step-by-Step Solution:
Step 1: Recall that mesoderm is the middle germ layer in triploblastic animals. Step 2: Understand that when a cavity is present between the body wall and the gut, its lining determines the term used. Step 3: Remember that a cavity completely lined by mesodermal tissue is called a coelom, and such animals are coelomates. Step 4: Note that Chordata, Arthropoda, and Platyhelminthes are names of phyla, not of the cavity itself. Step 5: Recognise that among the given options, coelom is the only word that denotes the body cavity lined by mesoderm. Step 6: Conclude that the correct term for a body cavity lined by mesoderm is coelom.

Verification / Alternative check:
Textbooks on animal classification describe three main body cavity types: acoelomate (no cavity), pseudocoelomate (cavity not fully lined by mesoderm), and coelomate (true coelom, fully lined by mesoderm). Examples such as earthworm, cockroach, and vertebrates are called coelomates because they possess a coelom. This consistent use of the word across phyla confirms that coelom is the correct answer for a mesoderm lined cavity.

Why Other Options Are Wrong:
Chordata: This is a phylum containing animals like fishes, amphibians, reptiles, birds, and mammals, many of which are coelomate, but it is not the name of the cavity. Arthropoda: This is a large phylum including insects, crustaceans, and arachnids. Its members are coelomates, yet Arthropoda names the phylum, not the cavity. Platyhelminthes: This is a phylum of flatworms that are acoelomate, meaning they lack a body cavity altogether, so this cannot be the name of a mesoderm lined cavity.

Common Pitfalls:
Learners sometimes confuse the term coelom with coelomate and may look for a phylum name in the options instead of focusing on the definition. Another mistake is to think that any advanced phylum name might be the answer because many higher animals have a coelom. The question, however, is asking for the term for the cavity itself, not for animal groups. Keeping the definition clear and separating structural terms from classification names helps avoid this confusion.

Final Answer:
The body cavity completely lined by mesoderm is called a Coelom.