Animals in which the body cells are arranged in two embryonic germ layers are called by which term?

Introduction / Context:

During early embryonic development, animal bodies form germ layers, which are primary layers of cells that later differentiate into tissues and organs. The number of germ layers is important for classifying animals and understanding their structural complexity. Some animals develop only two germ layers, while others develop three. This question asks for the term used to describe animals with two germ layers, so the learner must recall basic embryology and classification terminology.

Given Data / Assumptions:

• The focus is on animals that have cells arranged in two embryonic layers.
• These layers are outer and inner germ layers.
• Options include diploblastic, ectoderm, triploblastic, endoderm, and coelomate.
• We assume standard definitions of germ layers such as ectoderm and endoderm, and the term triploblastic for three layers.

Concept / Approach:

In animal embryology, diploblastic animals have two germ layers: ectoderm (outer layer) and endoderm (inner layer). There is no true mesoderm in these organisms. Examples include cnidarians such as hydra and jellyfish. Triploblastic animals have three germ layers: ectoderm, mesoderm, and endoderm, and include most higher animal phyla from platyhelminthes onwards. Ectoderm and endoderm are names of individual germ layers, not of the whole animal. Coelomate refers to presence of a true body cavity, which is a separate classification criterion. Thus, the correct overall term for animals with two germ layers is diploblastic.

Step-by-Step Solution:

Step 1: Recall that diploblastic animals have two layers, while triploblastic animals have three. Step 2: Identify the names of the two layers in diploblastic animals as ectoderm and endoderm. Step 3: Note that the question asks for the term describing the animals, not the names of the individual layers. Step 4: Recognise that diploblastic is the term used for two germ layer animals. Step 5: Select diploblastic as the correct answer.

Verification / Alternative check:

Classification tables in zoology textbooks list cnidarians as diploblastic and flatworms and higher phyla as triploblastic. These tables explicitly state that diploblastic animals have ectoderm and endoderm only, with mesoglea in between, and lack mesoderm. The term coelomate appears in separate discussions about body cavities. This clear separation of terms confirms that diploblastic is the correct label for animals with two germ layers.

Why Other Options Are Wrong:

Option B, Ectoderm, is only the outer germ layer and does not describe the whole animal. Option C, Triploblastic, refers to animals with three germ layers, which is not correct for the two layer situation in the question. Option D, Endoderm, is the inner germ layer, again not a term for the whole animal type. Option E, Coelomate, describes animals that possess a true coelom, a body cavity lined by mesoderm, and is unrelated to the number of germ layers.

Common Pitfalls:

Students sometimes confuse diploblastic with triploblastic or forget whether diploblastic refers to two or three layers. Others may focus on familiar words like ectoderm or coelomate without matching them correctly to the question. To avoid such mistakes, learners should remember the prefix di for two and tri for three, and associate diploblastic with simple animals like hydra and jellyfish that have two germ layers.

Final Answer:

Animals in which cells are arranged in two embryonic layers are called Diploblastic.